We could use several blacklists. Adding that list should be easy. Not that many of those words could be generated by our captcha, though.
I am worried however about your experience of «explicit words popping up too frequently». Could note them when you come across them? Maybe there's a pattern.
And of course, adding a blacklist brings out the old issue that the current captcha is completely English-centric (T7309). The Shutterstock list provided above has the bonus of being available in multiple languages.