DIAGNOSIS The spindle lock on the DCS570B is a little button you press on the top of the motor housing. If it's not engaging, it's usually because the blade isn't in the right position for the lock pin to drop into a notch on the arbor.
FIX Hold the blade guard back with one hand. With your other hand, slowly rotate the blade until you feel a faint "click" and the blade stops moving. While holding that position, press the spindle lock button on top. You should feel it click and lock the blade solidly. Now use the provided wrench to loosen the arbor bolt (it's standard lefty-loosey, righty-tighty). The lock only works in that one specific blade position.
IF THAT DIDN'T WORK If you can't feel any position where the blade locks, the spindle lock mechanism itself might be broken or jammed with sawdust. You'd need to open the housing to inspect it, which requires disassembly.
HEADS UP Never try to hold the blade with your hand or pliers to loosen the bolt. Always use the spindle lock. And remember to check blade rotation direction when installing the new one—the arrow on the blade should match the arrow on the saw guard.