One would think that adults know enough NOT to put their feet on chairs in a public place, especialy a place where people eat, but we all know that isn't true! I'd management handle it, or maybe put a bug in their ear, or send a comment, if you can, maybe asking if they can remind people not to do that.
At my PT job, we have a counter in the back, essentialy, with several chairs, which is the "break" area. its very tight, and we have 3 chairs there. I've seen on CW, several times, remove her shoes, and sit in one chair, and put her feet on the other. its not an issue of taking up two chairs, but more that her icky, perhaps stinky feet are on a chair we all sit on! I've never said anything as she's the type to take huge offense, but ewww. have some courtesy please! At home, on your own furniture, fine, but here, um, no thank you.