Posting nasty things on the internet is against the law in the UK and EVERYTHING you do on your computer leaves a record, even if you make up a name and use a hotmail or yahoo address. It's easy for the police to find out that you posted things on Bebo, Facebook or Myspace so if you're having a fight with someone, getting your own back could end up causing you a lot more trouble than you think!
If you have a mobile phone then don't make nasty calls to ex-friends, don't send rude texts and don't use the camera in your phone to take pictures of other people and then send them to other people or put them on the internet.