I had good results with doing an own small modification to our forum software. I had never done anything with PHP before, but knowing enough c programming that was still done below an hour (finding the right place to modify was the only hard part).

I think the point is not which modification I made - nothing that was much different from some other usual spam-prevention mods which are in use. The trick is that no spammer cares about a single site enough to work around a custom solution.