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A common characteristic of the recent high-energy (see MotivationEnergyAndCommunity) spam is inclusion of links to previously unseen domains. I propose a list of RecentLinks, like RecentChanges, which lists all new domains along with the pages they were first seen on.
Any link newer than the KeptPages expiry can be added to a veto list by anyone, which will automatically ban any further links to that domain (and optionally revert spam posts, if that proves feasible).
All vetoes can subsequently be reversed; however, the request to reverse a veto must itself survive the KeptPages expiry, and can be vetoed by anyone!
This feature could be abused by disgruntled citizens, but that would be at worst annoying, not crippling. It cannot, as far as I can tell, be abused by spammers.
This has already been done: it's called a DNS URI blacklist. Unless you want to claim that the management of the tens of thousands of domains that lists like SURBL keep is something that the Wiki community can or will want to manage, I would suggest using one of those lists, and having a supplemental "BannedUrls"" list of regular expressions matching URLs that could be edited by the community like any other page.
I work for a an antispam company -- writing URL blocks against spam is not just a full-time job, it's the full-time job of several people.