How to report unauthorized access of account?

[Saccharin]

Uh, that's an easy one?
Most people will use the same username for all.

Yes, would be my first try. Nope I dont use this as a usename anywhere!

Or better yet, if you use the same email/username and password combination in other sites, it was most probably obtained and disclosed by a hacker from sites with poor security. You'd be surprised.

I'd be very surprised if those types of 'hackers' would edit forum posts. This feels more like a prank by someone the OP allows access to the PC or account.