Set it to whatever pose and shape you want to freeze at and then from the mesh properties/shapekeys panel delete the Basis shape or if you have several shapes from the menu in the panel (arrow on left) choose new shape from mix, and then delete all the other shapes…
That’s if your shapekeys are relative. (if they are not relative post back)
I am assuming you did not make this rig so you would not know if there are bone driven shapes in it?