Alternatively, ensure you have enough legal disclaimers to disassociate your organisation from views of individuals, ensure bad posts are taken down as soon as a moderator sees them and then... you are doing "as much as is practicable" (sufficient legal precedent to quote that one.)

You will be able to have guests saying hi. You will not have to invest in IP / MAC monitoring, which is so easy to disguise there days that it is rather silly even considering them, and most importantly, you will be able to have differing views. And without that? Not much point in the site in the first place.

You could even allow somebody to say the government is not legitimate and get away with it. After all, it would be their warped imagination, not the considered opinion of a duly legal charity.