I had this problem with my Tensile cranks.
This is how I managed it:
Soak the pedal/insert in WD40.
Screw it back on the crank, and then tap the spanner on the pedal with a hammer/mallet.
Each time, tighten it back into the crank tighter, so the insert is tight in the crank. Then just keep spraying and tapping it. Hopefully the WD40 will soak through between the threads, and the shock from the hammer will loosen the pedal in the insert, instead of the insert in the crank.
I can't promise it'll work though, you might just end up loosening and tightening the insert over and over, and get no where.