It's been a week and you still don't have it apart?

You can grab the exposed end with a pair of vise-grips, or any decent pliers, or even hold it in a vice but that risks dinging the threads. Minor ding's aren't an issue so you might give that a shot. Or you might wrap the thread to try to protect it, but that reduces grip strength.

If it's stubborn and you don't want to ding it, take it to the hardware store and buy a nut to match. Stand the nut in a vise corner up, and saw it halfway through. Thread it on the stud and you can now clamp it in vise for a secure grip that won't damage the threads.