There is not a way to disable Prime Music without canceling your Prime membership.
The best way to make sure you always get Pandora is to us the app and start it from there or be sure to include "Pandora" in your request for music.
Keep in mind if you have existing stations, Alexa is very picky with what you need to say to get her to find it.
For example, I have a Pandora station called "70s Funk radio"
If I say something like "Alexa, play 70s Funk radio on Pandora" she has a hard time.
In order to get it to work every time I need to say exactly "Alexa, play the station 70s Funk radio from Pandora"