How do I cancel a booking that the client already paid for?

This is a situation that you need to report to Rover ASAP, not technically cancel

By doing so, they can have your back and flag her account so she doesn’t try this shenanigan on another unsuspecting sitter. She is trying to cheat rover too, which is why I’m confident they’ll be on your side of paying you even if services cease.

You can definitely use the “Trust &Safety info” sent by rover when it was booked.

9 canines + 2 felines + 1 aviary = a dozen pets ( not 1 which they covered)

Or you can contact through general support : Help Center Customer Support [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team]

I completely agree with Deb A. REPORT TO ROVER ASAP and have the additional charges applied IMMEDIATELY. There's no way in heck I'd tolerate that for even a second. I'm nice and I love animals, but I'm not being taken advantage of with that many animals.