Only thing you can do with an ip is trace it. If he is a very serious hacker he can flood you're I.P leaving you with using up 1GB or internet usage a day. But i must say, there are very rare cases of people being able to do that. You may know it as DDOS or something, forgotten the name TBH. Next time you log into the account, Get his ip, run it through an IP tracer, many free ones online.
Write down his ip. Can always help in the future.