Technically you CAN refuse to see an SP but as others said it is "polite" to give some money for the lost time/revenue. I guess the equivalent of a deposit at least these days (100$ for a 400-500$ indy). Now its not ideal to do. I wouldn't recommand it as you can risk getting blacklisted by agencies or indies (the words spread fast... they share bad clients, a lot of them are friends or at the very least aquintances). But if you really think its not gonna work, like 100% you want nothing to do with her, better do it than have an awkward session. I would suggest doing the best researches possible BEFORE booking. No idea on your frequencies, but as a ocasional "john" i can be "selective" in my bookings unless its a plan B last minute.
Now if its a bait and switch clearly then its all on her, not on you. Just turn around and leave, thats it. Its the equivalent of a scam. You are in your right to not proceed. She deceived you. They usually do it knowing well 80% of the guys will go trough anyway because either they don't want to leave with blueballs, or they are "too nice" to "hurt her feeling". But its your right and if its clearly a B&S.
In my years i would say there is maybe 2-3 times i think it would had been better to cancel the session, but it was not even because of look. It was the vibe. The girl was just not into me (or into her job as a whole) and i could feel it. Its like she really didn't wanted to be there. I eventually got trough all sessions because i hoped they were just shy. I would say in retrospec there is one i still regret having not cancelled because i don't keep a good memory at all of it, but the other, even tough she looked bored and was not into it, she was so beautiful not gonna lie i still fap to her pics to this day thinking "yeah ...i slept with her" ... lol. Sorry i know its not super class to say it but lets be real, im sure lots of clients do that
And actually the times i was "a bit" disapointed at first look, they ended up being awesome providers going the extra miles and that "disapointement" at the door oppening completely disapeared once i was making out with her and even more when she was blowing me giving me sexy looks

I came out with a big smile at the end.