usually what I have done with pipes that are seized is use some PB-Blaster or WD-40 and let the penetrating lubricant soak on the threads for a couple hours.
do you have room to get an adjustable(monkey) wrench on it?
be careful when you try using the wrench, make sure it has a good "bite" on the pipe before trying to loosen it.
also, you should put a "hold-back" wrench on the 90 in the wall if you have the room. otherwise you may damage the valve.
try the penetrating lube first and let me know how it goes.
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