I would propose that the first time someone does it, they're removed from the bus and forced to wait for the next one

The second time, they get a fine proportional to income

Third time, banned from that public transit mode for a few years at least

Doing nothing? These are not problems that require disciplinary action. Sometimes people will behave in ways we find somewhat annoying, this is just the reality of sharing a planet with 8 billion other people.