That sounds very annoying. Sounds like you need to start properly avoiding them unless it's possible to just tell them that you prefer to spend some alone time on your commute ( which I think is a reasonable thing to say straight to someone). Otherwise you might need to:
A. Get a bus from the next bus stop and don't sit next to them
B. Pretend you're on the phone to someone
C. Pretend you didn't see them
D. A mix of the above
Honestly it's so annoying, when I was still commuting I hated running into coworkers in the bus. Luckily most were of the same opinion and would just say hi and not engage further