When last I put my spare back, the plastic thing that's supposed to hold everything in place just spun the bolt in place and woudln't go down. Now, it looks like it's stripped.

I tried holding the bolt in place with channel locks and spinning the plastic thing, but it wouldn't move. Any ideas as to how to get this thing off? (Destruction is fine--I can always get another from the u-pick.)

 

Start off with a six pack.

 

Most likely Cross threaded which happens easily.
You may just need to brake the thing to get it off and out and then retrieve a different one at the salvage yard.
Grab a couple just in case it happens again.

 

Pulled the part off and threw it away. Got a replacement from the u-pull, along with the wingnut that was missing from my car and caused the whole problem to begin with. I reassembled everything according to the sticker, and now it won't go back together right because the tire is sitting too high in the well. HELP!!!!!!!!!!!!!!!!!!!!!!!!!!

 

Can you snap a few pics of how you have it together? Might just have something in the order wrong.