Use a different PC and change ALL your passwords to nice secure ones. He may have installed a key logger on your PC and is capturing your keystrokes.

Try changing the forum software you use to something else or make sure you've got the latest version of your current one.

Who owns the server that your site is running off ? You need to make sure it's suitably secure.

That IP address belongs to an NTL customer btw. You could try reporting it to their abuse team.