I am trying to change the blades on my Craftsman riding mower. There are 2 blades and I can get the right one off with no problem. The bolt on the left blade can neither be loosened nor tightened. Any idea on how to get it off? It turns like it's stripped, but I haven't touched it since I bought it new.

Hi,
I would say that the most probably cause is that the star-shaped part of the spindle that fits in the hole in the blade is stripped.
Peace,
Clarke