-Give them a hanging wedgie and get some rope. Tie their hands behind the back and if possible through the holes of their undies. Then tie their feet and knees together so the more they struggle the more pressure the wedgie has. If you have them as a willing captive, possibly have them have two/three pairs of undies on so they won't rip.(at least not easily) An alternative is to tie them suspended to a pole or on a pole and then give them a hanging wedgie off to possibly the top of said pole.
-Put them in a hogtie and stuff a dirty sock in their mouth before duct-taping and/or cleave gagging them. Afterwards, get some rope and tie through where you would yank to give them the wedgie. Afterwards keep looping the rope back and forth through the gag and back through the aforementioned section until it draws their undies up and their gag tighter in their mouth. The more they struggle the worse the wedgie gets, plus the tighter the gag closes to their mouth.
-Have them in a standing tie and have their hands tied above their heads. Have the rope leading through a beam above their heads, back down and through their undies in a way that any motion to try to bring their hands down causes them to give themselves a wedgie. For extra have their ankles tied together and have a way for them to have trouble staying balanced or would make them inclined to move their hands down(rollerskates?)