Why not encourage folks who don't plan on posting to watch the community versus joining it? I mean, there's nothing you can do to actually prevent it, but those who are considerate might consider leaving the actual community and just watching it instead. Just leave a bolded note somewhere in the userinfo.
Also -- is that really a 1000 friend-cap, or 1000 community member cap? Because the FAQ someone linked to above specifies journals, not communities.