I keep going back and forth on this issue: on one hand the spam is annoying (though it hasn't been unmanageable so far) but we're also linking directly to forum posting from Wikitravel pages and we don't yet have single sign-on. I feel like it's unwelcoming for new/potential users to get an 'access denied' page on their first visit.

Ideally, we'd have enough of a community to flag spam quickly plus some sort of blacklist / anti-spam filters or, at least, an approval queue like the one for comments. I'm still working on that angle on the dev site and will try to roll something out next week when I have a scheduled upgrade.

Now if the general consensus is that forum posts should be restricted to registered users, it's super easy to do. Just say the word...