This may sound stupid, but how could I forbid users to edit their own post in a forum? I'd like to allow users to create new topics and reply, but not editing their own post after it is posted.
If I set users' role as contributor, they can edit their post.
The easiest way would be to add an evaluator to the action configuration for the edit_post action.
Have a look at /projects/web-client/alfresco/web-client-config-forum-actions.xml for the definition of "edit_post", you could replace the <permissions> section with an evaluator (see other examples in the file) which is code based. In the evaluator you can programatically check the permissions and whether the post author is the current user.