On reviewing my own code, and thinking about it from a user-friendliness standpoint, I'm inclined to go for the latter. It's trivial to implement and easy to explain in the error message, in such a way that users will get it and passing spammers will not.
[Dan] Agreed! Would be nice to still be able to post links with a special tag. [rab] Can you get round your a link prohibition in the same way as at Dunx' place?
(sorry if that sounds a little curt - Dan's solution will probably be the workaround that will be offered if the server thinks you're being overly naughty)
On the topic of campus computer people: I have wasted an entire morning trying to get my laptop configured so it can use the university network and internet provider. The goons at the Computer Centre took about an hour and a half trying to do it before declaring that they didn't have the right driver for my make of computer. Assimilating, dubiously, the revelation that this varies according to make rather than OS, I then had to go to the other end of the campus (requiring not one but two bus trips) where, surreally, there is what seems to be a support place for Toshiba. Why this isn't next to the Computer Centre, God only knows. They didn't know what to do, because a look on the Toshiba website revealed that there are lots of possible drivers to download and I hadn't been told what kind to look for. So I had to go all the way back to the Computer Centre (through a monsoon), where a new set of goons had replaced the original ones, but were no more competent. They couldn't find the relevant part of the website and in any case were now talking darkly of the need to open up the laptop and fiddle about with its innards. They wrote down what they needed and told me to take that back to the Toshiba people. Of course, they are all closed now and I have to wait until Monday. All this just to log onto the wretched network! Oh, and the whole thing was conducted in Singlish, which is basically incomprehensible at the best of times but far worse when it's people going on about drivers and networks and suchlike. Really, this place is appallingly disorganised. Thank God I can use the university's own computers for internet purposes or I'd really be in trouble.