The GDPR will most likely not be a solution.
- Your posts by itself usually do not contain personal identifiable information;
- Even if they do the forum owner has a legitimate interest in keeping the discussion topic intact;
- And you have most likely given the forum owners a license to (use) the content you posted.
Personally I don’t completely understand how certain posts being out there on the internet suddenly become a problem as soon as you have a problem with the forum owner (or as soon as they have a problem with you). But I might have incomplete information here.