How can I remove the water pump pulley?

I think you might need this tool.. check your local auto parts store, they might have a loan-a-tool program..

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http://www.cadillacforums.com/forums/cadillac-deville-1985-2005-including-1985/466386-water-pump-pulley-tensioner-belt-removal.html

You indicated you were replacing the water pump and the pulley, so why bother taking it off of the water pump? Unless there are other issues like access to the pump hardware, just de-tension the belt and take the whole thing out.