Why?

I am so frustrated, and mad! In the last couple of months my kids have been stealing! At first it started with one piece to a game puzzle, then scissors, then I am missing a half a dozen white board markers. Why would they do it? What makes it even worse to me is that last month I made a huge deal out of respect. What does it mean, what does it look like, sound like, etc. Then if a student saw someone else being respectful that student would fill out a card anonymously, and at the end of the day I would read them. Once someone earned four, they could use my stuff again (after things started dissapearing they couldn't use my stuff anymore). So it was even more upsetting that after all this, and that every kid had earned the chance to use my things again that it was taken. I even gave the person a free card. I said, I don't care who it is, just put it on my desk the next day and everything will be forgotten, no questions asked. Of course, no markers turned up on my desk. Errr! I had to send a note home informing the parents about what is going on. I am also taking away fun things we do during the day until I get them back. Basically punishing the whole class until I found out who did it, or until they get returned. The one thing that is backfiring with that is we are reading the Constitution, and they brought up the part about having a fair trial, and being innocent until proven guilty. =)

Anyway, can you think of anything else I can do. I have no idea, and I am at my witt's end.