I propose one of two systems, the second of which uses the above as a backboard.
The I.G.N.O.R.E Cannon:A nifty piece of equipment from the NationStates forums, the IGNORE Cannon is a weapon that can instantly bot anyone from affecting a topic in any way! Simply take aim at the member OOC and fire, and they can no longer affect your topic in any way unless you decide to allow them back in! Obviously, this shouldn't be overused, only when someone REALLY gets on your nerves, as their own characters need to be able to do SOMETHING. Also, the IGNORE Cannon will not work if it is not your topic, or if the topic itself requires that member. Other than that, take aim and fire!
'Locked' and 'Open' System:Using my friend's PM system as a backboard, this system is very similar, and is implemented on many forums across the RPer communities. What it does, is if you decide that the topic is only for a few select people, then merely say, either in the first post or the topic's name, that it is 'Locked'. This means, that if anyone who is not allowed(people that are are detailed in the first post), wishes to join in, they must PM the author of the topic and gain permission. Elsewise, they are not allowed to affect it at all. If you do not wish to make only certain people able to contribute, then you may leave it 'Open'. If someone wishes to join, you, as the creator of the topic, may place restrictions, such as a limit to characters, so as not to ruin the story of it.
Well, whatdaya think?