You are talking about the plastic knobs that don't have a locking screw right? If so, a trick I've used is getting a couple credit cards, and slowly working the knobs up from each side using the cards.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
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Put a cloth on your guitar and use two spoons to lever up on the knob.
It's a fine line between clever and stupid.
Shove a heavy guitar pick under there. Always does the trick for me.
Best thing to do is put a cloth on your guitar to protect the body and it's finish, and grab a small enough screwdriver, and get it under, and pull up. It'll take it off.