How to punish kids for bad behavior ?

Well they are obviously disrespectful and the parents need to be apologizing to you and to take care of what their kids are doing. I would speak with the principal as well. Some things such as putting pins on your seat could be a crime. Maybe see what they can do maybe have some sort of suspension or detention or even corporal punishment. The little shits deserve to be hit with a belt it sounds like. See what discipline works as well and how they respond to it. Try assigning seats, see if there is some sort of thing where they have a card you sign, can you give them before or after school detention, have you tried talking to the parents of exactly what's going on, while some parents think their kids can do no wrong others will put a stop to it. Kids with parents that correct this sort of behavior are usually more scared of going home then the office. See if you can give them more work or harder work. Don't let them talk, take things away from them, make them sit or stand in a corner, etc. There will always be a parent that thinks that their little **** is a perfect kid who can do no wrong. But as long as you don't do anything illegal or against district policy or school policy you should be fine

Let the punishment fit the crime. Next time someone throws trash at you, stop the class, calmly walk over to the student, and inform him or her that you are escorting him/her to the principal's office. Then follow through and do just that. Once you have done that, go back to your class and resume the lessons as though nothing happened. If the kids act up again, repeat the process. They will catch on quick, believe me.

The pins on the chair thing is something these kids are doing because they think it's funny. You need to learn to inspect your chair before sitting in it, and if you find something, calmly remove it. DON'T react to it, DON'T complain about it, DON'T even act as though you know anything is amiss. Because the MORE YOU DO these things, the more incentive the kids will have to keep up their nonsense. When they realize they can't get you to respond to their stupid antics anymore, they will become bored and will stop the behavior. It's called being an ADULT professional and a role model, and NOT STOOPING to the kids' level.

One other note: I'm surprised that the principal of the school didn't ask for your resignation after you threatened your students with a belt. Don't EVER do that again, unless you want to lose your job, and possibly wind up in JAIL for it. Violence and physical punishments solve NOTHING. They just beget MORE VIOLENCE, and even worse, they send the message that the best way to solve a problem with someone is to haul off and slug them. HARDLY the kind of lesson that you should be teaching to young children.

As a teacher, you must discuss this with the Principal first.  Let the Principal make the decision for punishment.

If this was true (which I highly doubt) you should go to the headteacher, explain how the kids are behaving and get her to help you. Whilst your there I would ask for some training on how to treat a class of children which is not to threaten them with a belt. If you threatened my child I would not be sending an email asking you to stop I would be after your job, hopefully you are fired very soon. 

Make them go stand in the corner for their punishment

Give them after school detention

My old grade 4 teacher would make a kid drag their desk out into the hallway and place it so that he/she could see into the classroom and still hear what the teacher was saying, but would spend the class out there, in shame. Usually after the first kid had to go out there, the others would behave as they didn't want to be out there when the Principal came by, as they knew they would be in sht even further. Another thing she would do was a real stinker. She would write out a notice telling what the kid was doing to be in trouble, she would sign it, she would then make the kid take it to the office and have the principal sign it, and then have the kid take it home for the parents to see and send back. There was no getting out of it as the Principal was expecting that notice back with the parent's signature. That one was for a couple of the REALLY bad kids. Keep in mind I was in grade school back in the seventies, when parents used to discipline the kids and not the teachers. Good luck.

Send them to see the principal 

There is no way you are a teacher at all.  IF you were, there wouldn't be a question of what procedures to follow with insubordinate or inappropriate classroom behavior.  All schools have rules and regulations that are meant to protect teachers...which if you were one, you would already be aware of the protocol to follow.  This ends up being just really bad trolling since you've failed to do proper research before posting.   UPDATE:  All of your questions are epic fails.  

Punishment is a last resort. It is the refuge of the defeated. It is not the option to consider. Discuss this with other teachers...those you respect and admire. Let their wisdom be your guide. Meanwhile, it is okay to express your feelings. "Look, students, I am becoming very upset by this behavior." And look for the underlying causes of the behavior. Address the causes and you will stop the behavior. Likely they are bored. Redirect their attention to things more interesting than what they are doing.

Kids do stuff like that to teachers they don’t respect. Kids prank teachers they do respect, but in a different way. Kids don’t usually toss garbage at teachers they like. Punishment will not stop it. Try treating the kids like human beings and they might respect you a little more. Maybe don’t give out hours of homework every single night. It’s been a while since I was in middle school, but I remember the teachers I respected the most were the ones who didn’t talk down to me. So many teachers did. With teenagers, if you treat them like adults, they’re more likely to act like adults. And some teachers gave out an hour or two of homework every night. Multiply that by 5-6 classes, the kids have no time to be kids. A little homework is fine, but that stress can really take a toll on the kids.