First of all, no air-guns or paintball guns. Not unless you *really* like stink.
If you puncture a cat's skin within 24 hours the skin will close up and the pellet wound will fester into an abcess. If it bursts (and it will) that cat will bring the stink of Hades wherever it goes, especially under your house.
Smoke bombs, noise makers and the like probably won't work either. Cats will often freeze in place when terrified and run only as a last resort. Smoke bombs pose a fire hazard as well as stinking up the house (like spraying ammonia under there would also).
Go to your local pet supply store or check Petco online for sprays designed to repel cats. You'll need several cans and some old blankets, towels or baby diapers. Spray cloth until soaked, then place half-way over one of your crawlspace screens that is upwind of the typical kitty location. Repeat on several screens so the smell wafts through the crawl space. This stuff doesn't smell wonderful to humans either, but you can leave the house for a day and air it out later.
Once cats have vacated, repair the screens with new ones (Lowe's, Home Depot both sell snap-in screens in several sizes, just add a little epoxy on the edges for a permanent fix). Just be sure to inspect under the house before sealing up the last opening, otherwise you'll have to deal with the consequences.
If cats continue to visit, do try to trap them so they can be humanely taken care of, one way or another. Lastly, if you're trying to run a cat out of your garage, tool shed, etc... use a can of compressed air, such as "Dust Off". The hissing sound it makes sounds like a combative cat and usually scares them off. The IsoButane propellant smells nasty to them too, giving further incentive to leave. Just be sure you're not blocking the exit!