ok, first thing is first, have you ever had an account with secondlife ? ok can you not just change your password and personal information, that way he wont be able to get onto your avatar ? if you can get onto yyour account it might be best filing a abuse report against him outlining what he has acutally done, any proof would be great, you can also do this offline.
Check the links http://wiki.secondlife.com/wiki/Linden_Lab_Official:Sharing_or_transferring_Second_Life_accounts
http://community.secondlife.com/t5/English-Knowledge-Base/Filing-an-abuse-report/ta-p/700065
Hope this helps, and if you need anything inworld IM me Heaven Lacey