I think switching to vBulletin would be better as it supports a lot of plugins and has a lot of spam prevention addons.
vBulletin is proprietary and requires a $100 per year license. Of course, there are pirated copies floating around, and really, what are they going to do, threaten SR to take it down?
There are other more severe methods of preventing or in some cases completely eradicating spam, (especially when it comes to registration) but these would require users to loose some of their 'anonymity' - some high-end forums, and websites like Google.com etc., employ phone verification.
A simpler solution is to require a Gmail address when registering. Then you hand the problem over to Google's very effective spammer blocking mechanisms. Obviously we can't do that, though.
The easiest and most effective solution is just to have more moderators. Get more moderators who live in different time zones, which would be able to monitor the forum at certain times when others can't if their busy with work/college etc.
That has problems of its own, namely trust. That was the solution they tried a few months ago and apparently someone went rogue.
BTW, to anyone who says they don't care about the spam, if you are a vendor, then all the money going to the spammer is money not going to you. The $1000 that somebody sent the spammer a few days ago is a pretty big sale that some vendor lost. This potentially affects you personally and financially, so you should care.