Some pots have some thick grease-like stuff in the "bottom" (the casing), that creates physical resistance against the bottom of the rotating part, which I believe has an integral piece made just for that purpose. I've seen those types used on amps, but never guitars. Thick grease on the shaft may work, but it might be hard to get it in there.
Here's a trick I did on a strat's volume knob, because I kept inadvertently hitting it while strumming and turning it down. I used a rubber grommet, like the type that is used in a metal chassis where a wire is going to exit. What are they called...? chassis grommets...? If you don't know what I am referring to, picture a miniature bagel with a groove around the outer circumference. It had a 3/8" or maybe 1/2" inner diameter. I removed the knob, I sliced the bagel (er...grommet) in half, and put one half over the pot's bushing, resting against the pot's nut, then put the knob back on. Now there is some friction between the bottom of the knob and the grommet. You can vary the amount physical resistance by how hard you press the knob on - adjustable from very little resistance to very hard to turn. I put that grommet on my strat soon after I got it, around "92, and it is still working. I did try using a rubber band when I didn't have a grommet - wrapping an appropriate sized one around the bushing did accomplish the same thing, but rubber bands do not last very long before they disintegrate.