I was double billed last month for both buying and renting the same movie. If I bought a movie, why would it then allow me to rent? I never want to buy a movie. I never rewatch them. I'd like to turn off that feature. After chatting for 20 minutes to get my bill fixed, I was informed there was no way to disable buying. I searched the forum and this has been commented on way back in 2012. This is not very customer friendly. The buy option is the default option and it is easy to make a mistake or have a child/visitor accidently buy a movie. Please change rent to default or allow the owner to disable buying. Also, double billing people for the same movie is really annoying. Please don't do that.
There is a purchase pin you can add in settings under parental controls. Menu/Settings/System/ then go into Parental/Purchase that will at least add a step of putting in a pin before a rent or purchase happens.