It depends how cruel you wish to be, whether you want them to ever coming caving again and how well they can take a bit of pratting about but...
1. I once started to feed my legs into a badger set on route to the cave having convinced them the entrance was quite small. They were shitting themselves about following (I was secretly shitting myself there might actually be a badger in there)!
2. No knee pads.
3. Recommend they carry a banana (or something equally as messy) either in their pocket or under the cradle of their helmet for a snack mid trip. Just make sure they lie on the banana 'a bit'.
4. The obvious cave specifics like sending them the awkward way, whilst everyone else walks round or damming water and releasing at the precise point.
5. Emptying wellies into the entrance as they're climbing out.
6. Tape over their light so it's much dimmer than everyone else's ("sorry, it's an old light I'm lending you which isn't as bright as ours").
7. If you can find one, make them use an FX3 / FX5.
8. Wetsuit in a dry cave.
9. Get them to take their turn with the tackle sack having just filled it with rocks.
10. Hide their towel / change of clothes.
11. Very much cave dependent, but block the way out with a few rocks as you come in last, sent them first on the way back. Similarly add a few rocks to a squeeze for the return. Newbies, will never think to move the rocks!
12. Lie about it being a though trip once you've locked yourselves in / pull ropes down etc.
13. Produce a totally un-readable, complex survey which is vital to finding your way out.
I'll try to think of some other things I definitely haven't done to people in the past.