My kids can be like that at times. They don't get that bad very often, but about once a month defiance will set in and they will push the boundaries to the extreme (usually when I am already pushed to my limit trying to cope with other things.

They get seperated and put in their beds when they do get like that, until they have calmed down, apologised and are willing to behave again. My 6 yr old had discovered while I was pregnant that I couldn't physically force her to do anything, so she would lay down on the floor and refuse to move telling me 'you cann't make me', so I started confiscating all of her precious trinkets and toys, threatening to throw them in the bin or give them away if she refused to obey me.

My 6yr old nephew tried to pull a similiar stunt on me and my mum last week while we were babysitting him. Mum ended up having to give him a smack on the bum, because he got violent and started to deliberated destroy the bedroom and the stuff in it as well as screaming abuse. He's behaviour was really random - he had been good all day right up until that point and then just switched when he decided he didn't want his cousins near him anymore (he is an only child and not used to having other kids around for too long). His behaviour was worse then anything Ii have ever seen in my kids even, but then again, he seemed to think he would get away with it (and was very surprised when we didn't back down, whereas my kids 'know' that there are going to be consequences, and that I can be ever more stubborn then they are. My sister was appalled when she heard about his behaviour, but not completely surprised as he gets away with it at his fathers house apparently.