Ugh...Belts

Warning: Make sure you disconnect the battery first.
If you cant get the bolt on the end of the bracket (facing the radiator) to turn just take the bolt on the side of the adjuster (facing the washer reservoir) out of the adjuster out completely. The alternator now should be loose to pivot up and down. Remove the through bolt holding the alternator to the bracket (one attached to the head) and pull the alternator out. Then you should be able to remove the adjuster with the bracket, or have access to do with it as you please. I would soak it in penetrating fluid as the treads on the long bolt get corroded and create a headache. I have had to cut the bolt once and buy a new adjuster block and bolt from the dealer. I use anti seize since on every bolt since.
Good luck.

Getting the adjuster free is pretty simple.

Get the bracket out of the car (Two bolts on the block + the one on the alternator) and heat up that block with the adjuster bolt on it. It should not take much heat to bust it loose (Be careful, it does seem to twist off easily). Chase the threads if you got the taps and dies, and put the bolt back in with Loctite Never-sieze compound (I never reassemble stuff without it!)

There you go! It's a good idea to free it up as first, it makes installation that much easier, and second, it helps you fine-tune the tension.