Simple. Tell them that they're not one.
It won't destroy it, of course. Wherever a group of people collaborate for a common endeavor, there we find community.
But communities come in different flavors. My favorite kind includes a substantial amount of trust among the community members, and between them and their leaders/moderators. They are often powerfully goal-oriented, whether the goal is to build something or simply to have good conversation. These ones are electrifying to be a member of. Shared endeavors and a sense of shared ownership seem to actually create energy.
Other communities, however, just depress everyone. A group of rules lawyers, whose shared energy is absorbed in the feeling that bad behavior is punished but good actions go unrewarded, is still a community. It's just not a very pleasant one. One doesn't go out and evangelize for such a community or for what it does. One doesn't hope that others will come join it.
(There is a third kind of community to be mindful of, of course. A mob, like a depressive community, is a common failure mode of an energized community.)
The breakdown of trust is of course the most common reason that the first kind of community turns into the second. It's easy, particularly as a leader or moderator, to feel betrayed by everyone when the crowd goes in a direction that you don't want it to. And the fear of the mob is a powerful motivator. The temptation is to lock everything down, pretend that there is no community ethos but the one you provide.
But people don't work that way. Clamp down on a community, and it turns sour; the community spirit becomes one of grumbling and nit-picking conformance to the stated rules. Spontaneous action for the common good, being unrewarded, goes away.
I've seen online communities go completely sour at this point, as the members in their turn feel betrayed by the moderators. Subsequent events just confirm the mutual hostility. Eventually many of these things break up completely.
This isn't universal; sometimes the shared endeavor of the community is motivating enough to overcome the mutual mistrust. Gradually, a new balance is found; member behavior builds moderator trust and moderator trust reduces member resentment.
Communities may recover in time, but it's not a pleasant process.
This rather discouraged rant has been brought to you by the letter M and the number 2.