Blocking form responses that have inappropriate language?

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Our web site has a couple forms that allow users to send us questions/comments. On one of the forms in particular, we have a relentless "stalker" who constantly uses inappropriate language. Is there a form add-on that allows us to filter form responses? For example, if he uses a curse word, it wouldn't allow that form to be submitted. Any help on how to handle it would be appreciated. Thanks in advance!