Yeah, I clean it out periodically. The ironic thing is that because Gmail's spam filtering is so good I never actually see the spam, so I forget about it until I actually come to the Google Group website.
I don't think making a new list would be worth it because of the cost of having all the list members switch to the new list. It wouldn't necessarily be a huge cost, but it might be enough to dissuade people. On top of that, the spam would likely come back pretty quickly on the new list, so we wouldn't end up gaining much.