Anytime I’ve had a stuck one a strap wrench as shown in photo would be absolutely useless regardless of the direction it was applied.
Generally you need a lot more hold than that.
In extreme cases I have put a 3/4 x 3/4 x 3 foot bar in the jaws with the power plug out and belts and gearbox engaged to lock the spindle. A sharp rap with a dead blow or copper or lead hammer usually will start the chuck moving.
Use of back gear is not advisable to lock the spindle for this procedure because under these extreme conditions teeth can be knocked out of the gears easily.
a toolmakers clamp across a belt between the pulleys will often lock the drive solid enough.
Play safe
JD