I assume this won’t help if it’s a current situation, but it should be clearly spelled out in your listing and/or in your house rules the consequences of a guest bringing an unauthorized pet. If you indicate that you don’t accept pets at all, it is grounds for requiring the guest to vacate the property. If you do accept pets, spell out what your pet charge is. Other than adding the charge, it’s up to you whether you want to impose an additional fine for an unauthorized pet.
You might be thinking “But what about the bad review I might get?” Sometimes when you are new, you do need to consider the reviews, since with few reviews 1 bad one will influence future bookings. However, in my opinion this should never be a factor in the case of clear-cut house rules violations.
Airbnb should forfeit a guest’s right to review a host if a clear, proven violation of a major house rule such as this has occurred. Especially lately, there has been an increase in the blatant disregard of fundamental host rules. Review concern should NEVER dissuade a host from fair-minded enforcement of his/her house rules.