L.S.
You may have tried this already, but I have used a reward system instead of punishment sometimes. I have a 3 year old daughter who is as sweet as she can be, but she went through a time this year when she would bite at school, if someone took something from her, or if she got frustrated at someone. At first we tried the negative punishment approach, but it didn't do anything! She LOVES Reese's peanut butter cups, so I bought some snack size cups and each day that she goes to day care, I let her see me put one in her bag. I tell her that if she behaves at school and doesn't bite, then she will get her treat...this worked amazingly well, and now we don't have that problem any more and we don't have to do the reward anymore either! Hope this helps!