Next time I see someone with a bag on the seat next to them, I'm going to purposely sit next to them.
Seems aggressive, why do you want to sit next to them if there are other seats available?
I want to make them uncomfortable with my presence
I see that it annoys you that the person is taking up two seats, but you shouldn't sit next to them just to annoy them. Sit somewhere else instead.
But, what if I can't find another seat?
If you can't find another seat, that's one thing, but if there are other seats available, just take one of them. It makes a more pleasant ride for everyone if people aren't intentionally annoying each other.