My take on it is that except for a brief period while you are getting settled in your seat, your personal belongings do not belong on any other seat. I would be polite, but firm. "If you are not sitting in this seat, I plan to sit here. Please remove your belongings." If they say "I'm saving it," I would again firmly (but politely) say "I'm here first. Under the boarding policies of this airline, if you wish to board and sit with someone else, you are supposed to board with the higher boarding position Sorry. You can't save for someone with a higher boarding position." It really makes no sense that one person can spend the money, and keep someone with a lower boarding number from getting the seat over someone with a higher boarding number. The logic is ridiculous and you have to twist yourself into pretzels to justify it (general you, not YOU in particular....LOL).
Personally, I'd love to see Southwest be very clear about this and outright stop seat savers. It takes very little effort to announce "No saving of seats is permitted" and then the earlier boarding passenger would be less likely to do it, and there would be less disagreements.