If the site is doing well, avoid panicking - lots of webmasters freak out and go on a rampage trying to remove every link, when they should just sit back and relax. If we're talking thousands and thousands of junk comments, it's something you might want to be concerned about. If it's a lower number, I wouldn't lose too much sleep over it and would focus on building quality backlinks going forward. Contacting blog owners is a nightmarish task because the sort who have blogs that auto-approve junk comments are usually not the most tech-savvy and often don't even think to include their contact info anywhere. Lots of them don't own their own domain, so you can't get their whois info that way either. If it were me, I'd put my time and energy into building good, diverse backlinks rather than spending tons of time trying to remove these links that don't seem to have negatively affected you.
