How can I remove a tub spout with a stripped hex bolt/machine screw?
Assuming the inside of the hex nut is stripped so that the allen wrench turns within the nut...

Get a spare allen wrench and a replacement set screw and a small tube of epoxy glue.  Dab the glue on the end of the wrench and insert it into the nut.  Hold in place while the glue sets.  Unscrew the nut.

Discard the now fused nut/wrench combo and replace with new set screw using new allen wrench.