Members' accounts constantly being hacked (including my own) | Need help!

Well I have a clan site hosted on forumotion @ superunitedninjas.com .

Problem is our rival clan, @ http://bangbang.forumotion.net/ a member of it (Nick) has hacked our clan several times.

He added images of a penis, deleting accounts, changing passwords, etc. I've been trying to stop him but he gets in one way or another.

I've backed up the forum, and still it gets hacked. Is there anyway to stop this chaos?

Our last 48 users list is also bugged.

Edit: He got our members' passwords because he has his fansite, and some of our members are signed up on his site so he got the passwords that way.

Edit: Hacking has stopped.