too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
I assume you mean a brake with a recessed Allen key nut that is somehow rusted or otherwise stuck. If not, post more info.
The following assumes you have no wish to salvage the brake and just want your bike back.
The fork is easy. Using the Allen key hole as a guide drill down with a 5mm or 3/16" drill until you clear the nut and it simply falls apart. If you want you can look underneath to see how far that is.
Rear brakes are more of a problem because you won't have straight line access from the nut side. Disassemble the brake from the bolt (saw if necessary) then you have two choices. Either saw and file the spacer block until down to the frame then push the screw out. Or cut the stud off flush and drill down the center with a 1/4" drill through the spacer block, being careful not to go into the frame. If you drilled straight it'll come apart by itself, otherwise you might have to punch the bold out the bottom.