I check my spam filter (for my blog) every few days, I will look at the IP address if I see a common thread (such as 95.65.32.x today) I just straight out ban the whole region, my largest group ban was banning 112.111.x.x I am not worried about preventing people (that can read my site) from getting to my site. both of those IPs ranges are foreign (the first one is from a republic of something or other, and the second one is a chines one)
So foreign spammers go ahead and spam, I will just IP range ban you off my site.