Setting a hard ceiling for the number of answers for anyone is not a good idea, whether they are Blankman or Jon Skeet. We cap questions for a reason: to stop the help-vampires from posting 30 questions a day instead of researching their code. However, there's no reason to reciprocate this with answers; answers (I think) are valued higher than questions here.
If the user's answers are low-quality enough, they'll be auto-banned until he asks good questions and edits his poor answers. Alternatively, if they've copied answers from another user, you can flag them for plagiarism. Also consider leaving a comment on some of their answers and letting him know the problem