It's been my experience with the acorn nuts that they bottom out and distort the threads at the end of the stud. Due to inconsistancies of stud length, or how far they are screwed into the case, the acorn can tighten against the top of the stud if there's too much thread protruding from the cyclinder head. When this happens not only can the nut jamb on the stud but it isn't tightening the head against the gasket correctly.
Earlier, in a previous post I suggested you remove the cylinder head by removing the four nuts or in the case of the jambed one, the nut and the stud.
Several people have suggested alternatives to a vice; pipe wrench, vice grips, etc. A little scaring of the smooth portion of the stud won't matter, just protect the threads. A word of caution here; overheating can cause the metal parts to gall together making it harder to get them apart. Hold the stud by whatever means available to you, use a box end wrench on the acorn nut and force it off. A little back and forth action might help. like rocking a car out of the mud. You'll probably find the threads damaged a little, especially on the very end of the stud. These can be rejuvinated with the proper size/pitch threading die available at most hardware stores. It's probably an 8mm X 1 or possibly 8mm X 1.25
Or, you can order a new stud/s from one of the suppliers that sponsor here.
Just don't 'jack up' anything