If you can't tighten it with the tension nut or manually with a 2x2 as you mentioned, then the tensioner is shot. The tensioner has two main parts, the middle thru which the bolt goes and the outer housing. These two parts are joined by a rubber wall for dampening purpose. When this wall breaks there is nothing you can do, since the outer housing, which holds the pulley is free to move regardless of tightening the middle bolt. I once made a temporary fix (it takes a while to get parts out here) by drilling holes thru both parts and hammering in a few pins.

Vin