Memories.
My very first (and only pet growing up) was Butterscotch the Cat. He was a straight-up, outdoor Tom, who only got to come inside (thanks to an anal retentive mom) when it was like Zero outside.
Anyhow, we had a furnished basement with a suspended ceiling. Butterscotch would always find a way through the utility room up into the ceiling and walk around until he'd fall through one of the panels. One day after christmas when I was like 11, I was playing on the carpet with my new slot car track and the cat and ceiling panel fell right down on my head! It scared the crap out of me.
Cats are curious by nature and love to explore (<---that's not news.) Blocking off the access points will be the only way to kep Puss from wandering the nooks and crannies of your house. One little trick you can use is to put citrus rinds in the way of the access points. Cats hate citrus. Lemon, grapefruit, orange peels, etc.
MAY make them think twice about entering therein. The only problem is that indoors, they'll get rotten so you have to rotate out the rinds unless rotting fruit scraps are less of a problem than falling cats from the sky!
That said...unless you live in a colliseum, I doubt an acidental fall from a standard 8' or 10' ceiling will hurt a cat. Their pretty freakin' smart about
not getting stuck, too.
Look at it this way...at last you know there aren't any mice in your ceiling!