Do not hit the chuck or use an adjustable on the jaw.
Open chuck jaws enough for a piece of 2' or 3' by 3/4 or 7/8 square steel or aluminum barstock to lay parallel on one jaw and rest on the nose of the adjacent jaw.
Push barstock to rest on the face of the chuck.
Put lathe in low backgear and use the piece of barstock as a breaker bar.
Apply gentle pressure to the breaker, increasing in intensity until the chuck releases.
Leave no marks.
Don't practice lathe abuse.