What engine does the car have? I think most (if not all) 95 Centurys had a 3.1. Specifying engine size allows for a more specific answer to your question.
To remove the bolts from the pulley, first put the belt back on. With the belt on, you may be able to unscrew the pulley bolts. If the pulley still spins, you can try rotating the tensioner in the 'tighten' direction to get a better hold.
What I do to remove pulley bolts:
With the belt on, I get an open-end wrench 2 mm larger then the bolts.
I place the end of the wrench around one bolt. It will fit very loosely. I then use a ratchet and 6 point socket on a different bolt to turn the pulley counterclockwise until the handle of the open-end wrench contacts another bolt. Holding the wrench firmly, I use the ratchet to loosen the bolt it's already on just slightly, not more than a quarter turn. I then move the wrench to the next bolt until all bolts are slightly loosened. A second pass with just the ratchet and no open-end wrench then loosens the bolts one full turn. I remove the belt at this time and then fulley remove the pulley bolts.
If and of the water pump mounting bolts are stuck, do not apply excessive force and break them off. Instead, reinstall the pulley and belt. Then fill the cooling system completely and allow the engine to idle for about 15 minutes or until it is fully warm. You can now remove the belt and pulley, drain the coolant, and safely remove the stuck bolts without breaking them.