Stephanie We have spoken about this before, you need to speak to an adult or teacher about it, before it gets worse. Problem with kids is they think they are invisible online, and especially on MSN they are not!
Main thing is, DON'T hit back! Makes you feel nbetter I know, but it really does make the situation worse, so do it as you have so far, and either ignore or block. Its what they are there for.
There is a tab within MSN Messenger to report Bullying, Microsoft, the DoE & The police Take it very seriously, as do we in my role of work. It doesn't have to go unpunished, or even go on at all, if you make sure someone is aware of it.
Evidence can be gained from the Logs if it is done within School (doubtful if your School uses RM Computers) without his permission (under the Duty of care theory) so do it now.
Get more info and advice here (and please do it!):
http://bullying.co.uk/
http://www.besafeonline.org/English/bullying_online.htm
http://www.kidscape.org.uk/childrenteens/index.asp
http://www.getsafeonline.org/nqcontent.cfm?a_id=1216
http://www.whybully.co.uk/faqs.html