If you like to give ability to delete everything that is your choice. But it has nothing to do with copyright. It is right a content producer has anyway if that content is an unique work. You can’t even give away your copyright. It is matter of publishing rights.
When a work is published at a forum, creator gave consent for publishing. And that consent is really strong against creator and can’t be cancelled if it has mentioned on T&C.
Sure, there can be different laws depending country, but Bern agreement etc are really world wide.