Comment Partial Key Verification is your answer. (Score 2) 687
I found this answer on SO a couple years ago and flagged it as a favorite because I figured I might need it some day.
The short version is a lot like what people have already said, have cracked keys be detectable and then decide from there what to do.
This guy decided to redirect the users to a website to inform them that they're using a cracked key and that they should really purchase the software.
His studies seem to indicate that it works well.