Plastic Spindle Nut threaded

Sorry Incase this has been posted before . 

ive had the Same problems in the past that people seem to have with the power dropouts and right power sensor missing. I’ve managed to sort it somehow and not had the problem for the past 6month.

however, last week that nice little warning pops up “right power sensor missing”. Even with new batteries!
I decided to take the pedals apart and do the service Advisory on them. Watching the garmin YouTube video in servicing them. I done the left pedal .. nice and clean freely moving . I Didn’t watch the video past the demo on the left pedal. 
Comes to the right pedal and with it being 11:30pm I wasn’t thinking straight I’ve managed to over tighten the black nut not thinking about it being a reverse thread. The nut is just spinning while I’m holding the spindle not tightening or loosening . Is there a way of getting the nut off without damaging any of the main important parts of the pedal.

any advice or micky taking fire away ;)