To avoid catfish with live bait, the bait usually needs to be too big to fit into the catfish's mouth, and a mobile bait helps too. A 6-10" croaker, pinfish, mullet, or sand trout should deter all but the largest of catfish. However, I did once catch a 6 lb gafftop on an 8" croaker, so reduced odds are no guarantee.
Another trick to reduce ( but not completely eliminate) catfish is to use whole, small crab (alive better, freshly dead ok). Anything bigger than a quarter should reduce your rate of catching catfish. Another technique is to use cracked crab on a big hook (6/0 to 7/0). The hardheads will pick apart the cracked crab, but you can leave the top shell on and cut a whole market crab in half to slow them down. We've also gone to a 10/0 hook for cut mullet as bait, which deters most of the catfish from hooking up.
We've also done well using crab claws as redfish bait, catching redfish, but very few catfish (the occasional large gafftop will manage it). Hooking the claws can be tricky. We set the claw on a firm backing (boat seat or ice chest) and tap on the claw with a phillips head screwdriver (the pointy part that drives the screw) to make a hole on each side of the claw. The holes should be close to lining up to get them in the hook. An adult crab claw is a good match for a 6/0 or so hook. Punching the holes before trying to hook it is much easier than trying to hook a completely intact crab claw. You'll catch black drum too.
Finally, we recently completed a study where rare earth magnets attached near the hook/bait reduced the catch rate of hardheads by more than 70% in moving water. Our study is not published yet, but our method was similar to O'Connell et al (
http://fishbull.noaa.gov/1094/1094oconnell.pdf ). Note, rare earth magnets demonstrated no improvement in still water, presumably because water movement is needed to induce an electric field that is detected by the catfish.
Consider taking a few catfish home to reduce the population. If everyone throws all their catfish back and takes home specks and reds, which population do you expect to grow? Gafftops are excellent table fare. Hardheads can be ok.