This seems like an obvious way, but it is worth mentioning. I was reviewing the search phrases people use to get to my site, and I found some... extremely explicit ones. Not what you would expect on this site. Repeating the search in google brought me a to a post that had several spam comments of the more nasty side. That gave me the idea of trying:
I found a lot of those, sadly, and I was able to remove ~50 or so spam comments fairly easily. The main issue with spam is separating the chaff from grain, google made it very easy to find the bad stuff.