Fix with POP (Proof-of-Purchase). That is, you can enter the vehicle through any door, but you must have a valid ticket. Occasional inspectors check the tickets, and issue fines for people who don't have tickets such that the expected cost of not paying your fare is on the same order or lower than fare beating.

Put a fare vending machine which accepts cash and credits cards on the bus, which also makes it very obvious how to get a single fare. (This exists for example for trams in Berlin, modulo the accepting credit cards part)