Probably a number of different approaches you could take on this:
Attempt to parse out the contents of the "reply" editor form (not sure if its possible on the default built-in editor) and generate an email if a specific keyword (bad word list) is found.
Run a custom workflow that looks at the body of the response and if it finds a specific keyword, generates the email (probably a Visual Studio workflow - I dont think this would be possible with the simple workflows available through SP Designer).
Run a custom stored procedure on the SQL server that listens for an Add Record event on the database and runs through the reply data looking for a keyword, and if found, will fire off an email. In this case you'd store the list of bad words in a separate table that you'd run the comparison against.
Realistically, the third option is what I'd probably look into since I've used other "listeners" on the database in the past and have been successful at it. Of course you always want to keep in mind the idea that Microsoft does not endorse or support direct manipulation of the database, but in this case you're really just reading it...which in my mind, is ok (my opinion only).
Whatever approach you take, its an interesting endeavor - so good luck with it!
- Dink