How do I show I'm unavailable for dates where I'm already booked for house/dog-sitting?

When I look at your calendar, Todd, I don't see very much availability for any of your services for quite some time. It is odd that it says your calendar hasn't been updated for 2 months though. Anyway, you will have mark yourself as Not Available for each service manually. Just because you are booked for one service doesn't necessarily mean that you could not perform others. Many sitters will continue to perform walks and drop-ins while house sitting. Logically you can't board any dogs or provide daycare while house sitting, but Rover's system isn't that intelligent.

As for people contacting you, it is my understanding that past clients can always contact you regardless of your availability. If someone else manages to contact you for a service you aren't able to perform, then, yes, you can only turn them down.