I'm not going to allow a person I don't know to place his stuff on my seat on the bus.
How are you going to handle that?
I will tell him to move his stuff from my seat to somewhere else, or I will stop being polite.
Your seat is yours and personal space is important. Have a kind conversation with your shared traveler and set boundaries.
I'm going to threaten him with my knife if he comes near my seat.
That would be unwise, to say the least. That kind of behavior could be arrested and sent to prison for assault. Unless he placed his property in your space, you shouldn't be saying anything to about it anyway.