2. Create a set of rules for topic creator- must have to delete the spam within a certain time, must have to lock the topic after enough discussion.
3. If topic creator don't follow the rules, a temporary ban will be given.
The forum has some moderators and they are responsible for deleting spams and useless posts. Users should report the posts and the final decision should be made by moderators.
There are many topics created every day and each of them has its own creator. If topic posters are responsible for deleting posts, there would be a different behavior for each of threads, In my opinion all the threads should be treated similar to each others.