I'd like to remove the dial type thermostat (over 18 yrs old) from the wall in the house. Removed the only mounting screw I could find, but the unit still is secured to the wall, but can be rotated 90 degrees either direction. I cannot find another screw and cannot pull unit from wall.
How can I remove this unit?

Some of the older style dial units fitted into a subplate. Rotate the t'stat 90 degrees until it stops and try to wiggle it out of the mounting slot and off the base. Underneath you should find 2-3 screws holding the base to the wall.