What are a commander's options if SM fails numerous promotion boards, still has a service obligation, and he does not want to deploy to NTC?

MSG (Join to see) I think you know the answer already.
Task, give lawful orders, inform, and remind the SM, That the uniform tag opposite their name tag, the US ARMY one....
Also stands for Uncle Same Aint Released Me Yet.
Do all that while recording his successes and failures...praising the one, counseling both as needed.

He has only two choices...well Id only offer two..... giving in to his wants by way of a tantrum was not my way...

So the SM can Soldier on till terminal leave date.
OR
Soldier on though Extra Duty and daily duties assigned until his Terminal leave date.

And YES he needs to go to NTC.. even if you don't use him in the unit line number assigned position... the unit will be tasked for more then a few SM to do "other duties" Would you rather pull a good SM from the platoon to clean latrines and dump trash cans, sort trash for ammo, guard the wash bay, clean the wash bay oil/grease pits? Or say....Someone who needs a reminder USARMY

The first thing to do is bar him from reenlistment due to repeated failure of the promotion board, and lack of promotion potential. The second thing is to issue formal orders for movement.
It sounds as if the service member believes he can be a great burden on the unit if he decides, and that it is in your best interest to leave him. This would be the time to educate him that he can leave the army with a less than honorable discharge, paying back his bonus, or student loan repayment that he likely has, and any GI Bill he may be entitled to.
Usually, when the commander is faced with one of these guys, they basically come to the agreement that as long as the service member continues to perform they will leave with their education benefits. But the first slip up and the service member will find themselves with an Article 15 and a further degraded discharge.
The service member will probably never be a contributing member of the military or any leader. But, if you give them the two choices, the carrot or the stick, they usually take the carrot. The carrot is they ETS honorably. The stick is taking all their pay, their rank, their time, and their benefits.

Didn't see much from the Skipper community so I'll relate how I operated. My CoC (portion that drills directly down to the sore spot) was always on the same page or the XO and SEL would be in my office about it. The notion is to identify early, process to a conclusion quickly, and move on with life. Decision is with the SM who enjoys or suffers the relevant consequences. If the SM insists on a Bitch at the CO Session, it happens quick and I force a decision which, depending on the direction, takes about a week to go sufficiently downhill and typically longer to climb up. Keep in mind that good Skippers make business decisions, not bleeding heart decisions. We have too much mission, limited resources, etc. to be a campground for aimless wanderers. The "set the example" notion shouldn't be the primary focus because that just highlights the badass aspect which you don't want to be ever prominent. By working as a CoC group, you demonstrate to the rest of the troopers that you are effective and focus on the right things, including bringing them home alive. Good people don't like to be dragged down and the more you clean up their working environment, the better. Been there, done that. The exMILs who likely still hate my guts can hate all they want. Just not in my Service.