Most folks would engage the back gears to prevent the spindle from turning, but this can result in breaking a tooth off one of the gears if too much force is used; personally, I put a wood block between the rear bed way and, and with the back gears engaged and the lock bolt withdrawn and the spindle speed set for the lowest speed, reverse the lathe so that one of the chuck jaws contacts the block of wood And generally, the chuck starts to unscrew and can then be removed further by unscrewing by hand; this method, too is not without risk to the drivetrain; I think there is no easy positive way that does not carry some risk of damage. Penetrating oils will prove nearly useless, as it is the fit against the shoulder of the spindle that is causing the difficulty, and no oil is going to penetrate a metal to metal fit.
You might first try the previous writer's method with the rag to jam the belt and the tapping, then if that is not successful try a good size Crescent wrench on one of the chuck jaws and give a number of good sharp raps at its outer end.