Recently a new user's account had to be deleted after he spammed the site three times in a few minutes. Do we have measures to avoid that he creates a new account and repeats this? Like for instance a blacklist of sites you can't link to in questions or answer?
edit
I just realized that blacklisting the complete site may give problems with sites like http://www.tinyurl.com.
Jeff's answer covers the tools the engines give us fairly well. Here's how to use them:
Just flag the posts as spam, and downvote.
If enough users flag as spam ('enough' == 5 for now), a temporary block happens automatically. There were 7 flags across the three answers and user account itself when I logged in a couple hours after it happened. When a mod logs in, of course, we can take immediate and permanent action.
In this case, I don't think we need to bother blacklisting. This was the first time I've ever seen spam for this particular site. If a new account is created to spam the same site, we'll put it on a blacklist.
Let's see:
se 2.0 sites do not implement the question rate limiter (6 Qs per day per user, 50 Qs per month per user). Doubt these were questions; spam is almost always in the form of answers.
the standard rate limiters of 2 minutes per answer, 5 minutes per question are still in place. You can't post questions or answers faster than that, at least, not from the same IP address or account.
there is auto-flagging of duplicate answers, but we do not block them -- we do block duplicate questions. That's assuming the user had similar enough text posted for each answer for us to detect it.
of course we can blacklist URLs but most spammers have URLs so disposable that by the time we blacklist them, they'll have tossed it for another throwaway URL. (but if you see repeatable patterns, let us know.)
for new users, if a LOT of your posts are flagged, deleted, downvoted, etc you will be auto-blocked from answering (or asking) any more. This happens fairly rapidly, but takes action from the community and mods to kick in.
