How to stop the spam adverts

[Apatheist]

Sadly, we have recently been attracting some losers who think it's worthwhile spamming us with adverts for other sites.
How can we stop them?

One could perhaps deter the spammers by auto-removing any external links, but that would reduce the general usability of the site: some of the forums rely on genuine external links for the discussion - Acceptable Content for example.
Is that a price worth paying? Or is it sufficient just to report any spam that turns up and have a mod do a manual edit (thanks TeeGee)? Is it possible to distinguish (and keep) internal links (games, quotes, users etc)?
I would hope that the minimum punishment for spam was instant removal from the site, although that doesn't seem to have happened in every case yet.
For some reason the number has increased recently.

Other than banning external links, the only thing I've thought of is preventing posting anywhere on the site until having completed a minimum number of games or a certain amount of time (spam posters invariably post immediately after joining).

Any workable ideas?

[TeeGee]

I've been taking (or trying to take) a bit of downtime, will be back in a week or 2