replying to John Keiser, Hackin' through wrote:
I just had two faucets with the same problem - hot water side on both, a lot of
calcium carbonate deposit. Soaking in vinegar seemed to have no effect. Then I
1) sprayed on some liquid wrench, 2) waited an hour, 2) put in a _high quality_
hex wrench (a bit-type that fits a 1/4 inch drive), 3) tapped it a few times
with a hammer to try to vibrate-out some of the stuck-ness, 4) put some pressure
in both directions a few times (clockwise/tighten and counterclockwise/loosen),
then 5) went for it, to loosen.
Sink #1: The set screw came out, nothing damaged.
Sink #2: The high quality hex bit broke in the set screw. The broken piece stuck
out enough that it was easy to extract. Back to the hardware store to take
advantage of the bit's lifetime guarantee!