You have to keep beating the gong. In our description it says “all rental times must include load-in and clear-out.” And I repeat it when they book–in the nicest sort of “make sure that you check with your caterer to ensure that if they arrive to set up at the start of your rental they will have enough time for their prep.” And then, when the caterer is chosen (we request contact info for the caterer) I call up–if they don’t reach out to me first–and point out that they can arrive anytime after the rental starts. This puts the onus on the caterer to call up and say “I’m sorry, we can’t do a sit-down-dinner for 50 if we’re only allowed in ten minutes before dinner.”
Most people genuinely forget that an event takes time and setup. I’ve had people say “so I figure I’ll drop by at 4…” when their rental starts at 6, and I’ve had to remind them of the rules.
Sigh. As Paul says above, clarity is key. If you have to say the same thing fifteen times before the event, that still saves having a scene during or after the event.