I don't think just stating that will work, people will just ignore it.
Why would they ignore it? If they do that's grounds for sanctioning I'd suggest. Or merely having their posts deleted.
Equally even people who I have little respect for and caused problems previously have on the odd Occassion contributed constructively to threads. So let them contribute and see how it goes. If they behave, no problem, the moment they act up they get switched off by the OP.
Dynamically controlling who's able to post looks to me to be more problematic - does the OP maintain a list in the first post? If so then likely candidates for being excluded would need to check that list prior to making each post. Seems cumbersome to me, and a disincentive to posting.