I echo trueblue's comment 100%. If you are planning in advance, maybe a sign that you can leave on the kayak stating that the mooring is already paid and you are just out for a day sail would help keep the overzealous sailor off it. IMO once you pay, the ball should be yours regardless of if you are on it or not (as long as you leave something tied to it). That said, some charterers don't make the most rational decisions when they pull into a mooring field at 4:30 only to realize that there are no moorings available and they've never actually dropped the hook outside of their basic keelboat class.