You can ban a player from posting in your threads by blocking them, and you can delete your own individual posts but deleting topics altogether just because you've started them, and the ability to delete other people's contributions to a topic just because you started it are not available. The latter would just be a disaster in terms of how open discourse took place here and the former was changed in recognition of the premise that once a thread has been contributed to it's owned by the community, not the thread's creator.
If you have an issue with a user in particular you can always report it as abuse and let the forum moderators deal with it on a case by case basis.
I've posted a few topics and been a bit surprised to see that people post spam in my post. It's sort of nice to see it get bumped to the top, but I think the forum creator should be able to ban a member from posting, or delete posts that they have posted that are not relevant.