I recommend lowering the knob so that it rubs against the pot nut, making it harder to turn and shouldn't move on it's own.
Some customers also put a felt washer between the knob and the nut to increase physical resistance.
Our knobs have little set screws in them loosen this with a 1/16 hex key, lower it down as far as it will go and then tighten it again. You're done!