One fairly sure-fire way to take the wind out of a bully's sails is to simply not react which is particularly easy to do online (less easy or effective with in-person bullying). Online bullies live for the reaction, the defensive response. If you don't react, they've lost their audience and will move one to find someone who will excite them with a defensive response. You do tend to respond and defend rather than ignore, and that may be why, online at least, it becomes a problem for you.

If a forum is moderated, you can choose to not respond to the "bully" directly, but rather, bring the post to the attention of the moderator to handle.