What are the chances of damaging the ribs if I try removing the spar with a debonder??

If you take your time and let the debonder do the work, it can be done. It may be tedious. You need to decide if it is high enough to cause a problem as it is now.

Just sand it, I wouldnt waste the time removing it. You said it was about 1/64th. 1/64th off a 1/4 or 3/8 piece
of balsa is not going to hurt it. 80 grit should make quick work of the 1/64th of balsa.