To test that, try joining that other forum you mentioned with either a different user name, a different email, or both.
What you suggest can be the reason that could get me into this trouble. Once I couldn't wait for confirmation email and created in short time another account with different name and different email (fake antispam alias) to check if its not the case of blocking me, obviously from the same ISP and probably from the same IP (even though I have provider giving dynamic IPs). Meh...

don't really want to discuss this topic anymore if that could give me even more trouble...
I can contact the developer of the forum plug-in software about how to get you off the list, but they may just be tapping into yet another person's database. That may complicate things. But I'll certainly see if anything can be done.
I'm not sure if I want to do that then

Definitely I don't want to change my email address when it will somehow be blacklisted forever. I've been investigating if anyone could get into my email, but it's very, very unlikely... I access my inbox via providers' www, haven't got problems with viruses for several years, change password every few months...