There are two ways to do this, either by war, or by influence. Giving these to Myanmar will give Pakistan influence, and unless you want Pakistan to declare war, this is the only way forward. Not selling will simply push them to find another supplier, and they'd continue on killing not worrying about any foreign pressure, especially from Pakistan.
@Horus is right, this is how geopolitics works. Either Pakistan pulls a NATO and bomb Myanmar, or Pakistan gains influence and uses it in a productive manner.