I can speak as an affiliate and program owner as well...
The reasons why someone didn't make any sale could be millions, especially among small affiliates (i'm one of them), so i wouldn't really decide considering only the number of sales or the number of visits the affiliate sent.
Best way would be to send a nice e-mail, asking if you can do anything to help them in promoting you. That's actually something i would surely appreciate from a program owner.
Of course after that e-mail...if things don't change in a month or so...you could surely send another "less nice" e-mail and then...just delete the affiliate
