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You cannot unfreeze a compressor. You could remove the a/c compressor and put a smaller belt on to resolve your concern.
Once the a/c freezes up it will never turn again.
Hi Im Scott
Once the Compressor has seized and locked up like you say yours has it is not possible to free them up as usually this is as a result of an internal failure not physically allowing the compressor to turn.
You have not mentioned which make and model of vehicle you have, some systems run what is known as a serpentine belt which I am guessing your is. If this is the case it is possible to purchase a shorter belt which will allow you to bypass the A/C compressor.
If your system has a separate A/C belt then its as easy as removing the belt.