Cover the rim all around the valve with a thick layer of duct tape until the tape is almost level with the exposed surface of the valve. Then put one more piece of duct tape over the top of the valve.
Draw a line across the top pf the valve with a marker pen.
Get a miniature hack-saw and very gently start cutting a groove down into the top of the valve (perpendicular to the rim). Use a competent friend to hold the wheel upright and very still while doing this (may even help to add more duct tape to hold the wheel down on the floor). Once the groove is deep enough to hold the blade, remove enough of the duct tape so you can see how far down the rim is.
Keep cutting down to the limit of your confidence (i.e. don't cut into the rim).
Now use a thin-ended screw-driver to unscrew the valve. If the cut is to narrow to easily fit your screw-driver into, use a narrower blade (like a bread knife) until you have the valve down almost flush with the rim.
If that is all too scary, take it to your LBS.