The thing is, your passive-aggressive stink eye only empowers this wankery. Those folks know it's more uncomfortable to ask them to move their bag. You need to be blunt - a "I'd like to use this seat. Could you move your bag?" is plenty straightforward without being a dick. More people should try it. Same goes for airport gate seating!!
No, I'm going to make you ask me to move my bag. This being on the bus, the odds are pretty good that you're sweaty and reek of BO and/or ungodly amounts of cheap perfume, are irritatingly talkative, or in some other way an undesirable seat partner. My bag is there as a self-defense mechanism, if you're too socially awkward to ask me to move it, eat shit and keep moving down the aisle with your unbearable eight-hour workday.