They'd most likely have to do it through bans in reaction to non-vegan sounding comments, since there's really no way to prove someone is not of the group, until they prove that they're not in it.
It sounds rough, you'd need to have a very active group of moderators.
They took over the vegan circlejerk subreddit after I left reddit and don't know them, they run a mastodon server that I don't really understand and seem to be using bots to bridge reddit posts. My instance is bot free.