Disturb by a crazy guy.What should i do to stop him?

I know this clmate(guy) for about 7 years and recently he has been disturbing me.He haven't been so irritating in the past but he recently he start to steal the girls hotmail name and add them and he just keep on disturbing us.Like e.g,he ask us "what are you doing now","You guys just dun understand me"and a lot more.His been a quiet boy for the past 7 years in school but i just dun understand why the sudden change in him.He likes to view our blog and give comments when we didnt even invite him.Just a few days ago,i dunno how he manage to get a friend(girl) of mine and ask me to go out with the two of them.My friends told me that his a pervert and i should stay away from him thats why i didn't agree to go out with him.But a few minutes ago,he met me in msn and start to pester me to give him a reason for not going.I dun wish to hurt his feelings that why i just reply nothing.But i think his not going to let me off so easily.I still need to attend sch with him for 2 more years!!