Tornadoes meeting with an Immovable, Unbreakable, Large Cube
It depends.

Take a look at this recorded presentation from the 27th Conference on Severe Local Storms by David C. Lewellen and the accompanying pre-print.  This talk focuses on the effect of buildings on tornadoes though the buildings are examined as fixed blocks they are not modified or damaged, but remain a fixed obstacle to the tornado (he toward the end does show that he models forces of the tornado on the building but does not allow that to change the building).  

The effect of the block depends on whether the tornado is high or low swirl, the relative position of the block to the tornado and the size of the tornado.  The block weakened some tornadoes and strengthened others.