One of our customers has a great solution to this problem. If you wrap rubber bands at the base of the pot it makes it harder to turn.

However, this isn't ideal since the rubber bands can loosen the nut.

I recommend lowering the knob so that it rubs against the enclosure, making it harder to turn and shouldn't move on it's own.

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!