Attacked by Porn

Seems like my site is being attacked by porn. It started the other day with 2 new comments that appeared in really old posts, and linked to porn sites (if you click on the link for the name “Lolita”). I deleted the comments, only to have them reappear the next day at the exact same places. Today there was a whopping dozen or more, all in the comments section of various old posts. I can’t block the IP address as it always changes. Any suggestions on what I should do. Doesn’t seem too harmful yet, but I wouldn’t want to see it get worse. I guess that’s the price for having a certain amount of regular traffic.