I was doing that too.
Just now I thought why am I deleting these "very kind" comments and I approved one spam comment and then
- went to the page where it is,
- then clicked EDIT,
- then removed his/her website
- removed few words of bad english,
- added within context my keyword
- clicked save
P.S. I am still a newbie so if this is some kind of a major SEO error please correct me.