Some ways to deal with this:
1) Consider it a "physiological" stenosis, which varies depending on the use or stretch or turning of the muscle, so that flow sometimes can pass and sometimes can't. Leave it alone.
2) Stent it from the inside. The muscle can't compress a stent. Not the best idea because you're trying to address an outside-the-vein problem from inside the vein and you can introduce problems within the vein caused by the stent.
3) Cut the muscle, as you suggested.
4) Use botox on the muscle, see if it helps before doing anything permanent, or have repeated botox be the "permanent" solution
4) Use neck stretches to lengthen the muscle and relieve pressure on the vein