Reflections in glasses can be a real pain , so ask your model if they always wear glasses as many people will just take them off.
If they dont , try getting them to angle their face slightly up , down or to the side and watch to see if reflection disappers. You may have to change your position in relation to the light source , too.
No luck? then ask them to lift the arms of the glasses up their head slightly downwards . Or , if you are using off-camera flash , raise the flash gun up slightly and angle it down.