Ive been tracked on the net before by tracking cookies, and had virus attacks, thankfully not on the forum.
If you have a subscription internet security program you might also have some thing called identity safe offered by the subscription program, not saying this is the case, but if another has this pass word, then when activated your systems program might fill the login fields, this means unattended anyone with that pass can access one or multiple accounts or sites your a member of.
To hack your IP address then start posting as if they where you is very odd, as any individual stands to gain nothing from this type of action, i think i understand what your saying, but really dont get whats to gain from a hacker doing this, IE there is no financial gain from someone impersonating you.
Most identity hackers target financial accounts, not forums, very odd.
Dave.