Fare evasion can only be prevented by tough Fare Inspectors.
Fare inspectors have to appear when you don't expect them.
Their job is to thoroughly harass and embarrass anyone who didn't pay.
Here's how I would deal with people who are caught evading payment of bus fare:
In all cases, the offender is taken off the bus at the next stop, or is prevented from boarding.
He stays with the inspector until he has chosen one one of the following ways to pay:
Note that in both these cases, the offender has paid a surcharge for getting caught, and for the Clipper Card. He may now board the next bus honestly.
Maybe these charges should be larger, to cover the whole cost of the fare inspection and offender prosecution. But I think the scheme would work.
The charges should be very simple and have a high deterrent value.
Note that all fare evaders do not get a youth discount or senior discount. They pay full adult fare.