I'm hearing that all-too rare word floating around in the background.... "courtesy".
If you don't want people to be unhappy with you for leaving your bag for hours on end if you're not at the palapa/cabana,
just don't do it. Again, think of how you'd feel if you had to walk by again and again seeing it "occupied" with nary a soul in sight.
Set the example, and if you need to, politely mention it to someone you see doing it.
If people don't realize what they're doing, and they often don't, they don't know to change.
And if they do realize and just don't care, well, hopefully they'll be going to a different version of paradise than mine...
as in another resort.
I do however hold out hope for those who monopolize space on the beach... happy shopping to them.