Hi Lumberjack,
It's usually down to the terms and conditions of the forum or site concerned. Many will include a boilerplate clause which requires the contributor to agree to grant the site's owners a worldwide and perpetual non-exclusive licence to use their contribution as they see fit. In a case like that, in theory the licence is irrevocable unless it can be clearly shown that the term was
unfair, or that the contributor was not made aware of the terms and conditions before joining. A term which an average person would think was reasonably standard in such circumstances would not usually be judged unfair. This is a matter of contract law rather than copyright law, and so I won't spend too much time explaining the finer details here.
Some more enlightened sites provide a sunset clause which ends the licence when a subscriber cancels their membership, such as this one from Facebook:
For content that is covered by intellectual property rights, like photos and videos (IP content), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.
Suffice it to say, if there are no specific terms and conditions attached to joining a site, there will be implied licence that the site can publish and use the contribution, but that if a contributor then wishes to withdraw their posting, the site's admin should comply.
The policy on this site is somewhat simpler:
The materials on this site are subject to copyright and are protected under UK and international copyright laws. The copyright belongs to Copyright Aid and other contributing authors.
