Leverage is the key, you need something you can get good leverage on. I use a pry bar on the nut holding the pulley on using the nut holding the tensioner to the engine as the pivot point, remember to loosen the nut holding the tensioner to the engine a bit to allow it some movement of the spring, not a lot mind just not locked down tight.
Surely the the nut holding the tensioner to the engine should allow the tensioner to move otherwise it is not doing its job. if when it is tight it does not allow the tensioner to rotate against the spring pressure then the unit is fubarred. Or am i misunderstanding what you wrote?
mine rotates when the tensioner is at the correct torque.
Cheers