Should I modify the service and give them a refund?

Have they asked you to do so? Also, since you didn't sign in with your Rover name, I can't see what cancellation policy you have set and when they would have been required to contact you to cancel any date(s). Also, it doesn't sound like they made any effort to keep you in the loop.

Wherever you are in the country, this cold wave and snow storm was well-publicized in advance.,

IMO, unless these are great clients of yours, I wouldn't initiate anything. Moreover, you have to view this as a business; you are not doing this as a favor for a friend or relative. Their booking meant that you couldn't accept anybody else's. So they do have to pay for that guarantee. Let's say your clients were headed to a hotel in Hawaii. The hotel held their rooms despite not checking in. Your situation is really no different.