To find out _who_ it is,
you first have to identify the owner of that IP (the ISP),
then you can ask them politely if they will identify who was using it at that time.
(note, they will never give it to you if they follow the law).
If you just want to identify the IP, and who that owns/manages it,
you can as a starter go here :
http://www.ripe.net/fcgi-bin/whois
and enter the offending IP.