I suspect if it's the only supply of water available to them within a reasonable commuting distance from their nest, you're probably not going to be able to stop them from coming to visit the water fountain to drink.

I'm guessing your water fountain uses some kind of electric pump to circulate the water so I'd be careful and not put any spray or chemicals in it that would corrode or otherwise cause damage to the pump.

To help get rid of the wasp in your immediate area, you might Google 'wasp bate trap'. I seem to recall one that uses a plastic pop bottle and bananas as the bait that supposed to work pretty well (I've never tried this myself).

I use the bait bottle and it does work. Have got lots of them this year already. I use sugar water with any fresh fruit like banana or watermelon.And I also have quite a few waspinator nests hung in my trees.