I cannot remove my 498RC2 Ballhead from my Manfrotto 055 tripod. I have removed the screw on the underside but cannot remove the ballhead. It just spins merrily around.
If the 055 series models are anything like most Manfrotto products there are three small "grub" screws with Allen keys countersunk in the bottom of the plate on which the head mounts. All three must be loosened to allow the head to be removed. If you only removed a single screw, it sounds like you may have removed the one with a large handle that allows the center column to rotate.
If you've backed out all three grub screws and the center columns mounting plate is spinning then it is loose in its connection to the center column. You'll probably need to use a vise or other tool to hold it stationary so that you can twist the base of the 498RC2 off of it. You can use felt cloth or something similar to prevent the jaws of the vice from marring the finish on your mounting plate.
For a more comprehensive look at how set screws are used to secure heads to various makes of tripods, please see this article.