I think this will work out great for them. Somethingawful.com is one example of a site which has a pay-barrier to it's forums primarily for the reason to keeping out trolls. Now you may have mixed opinions over the nature of things discussed in the forums there, but the maturity of the discourse is pretty impressive for the size of the audience. The fact that so much of the topic matter is 'immature' only helps exemplify how effective the system is.