I'm assuming that rust and/or mineral deposits are making removal difficult. You can try the brute force approach of using a large wrench and turning it. Or you can apply rust or calcium remover to the threads and then use a wrench.

You can try [pickling] vinegar: saturate to run-off a cloth or sponge and wrap around the joint, then cover with a plastic bag. Check it in an hour -- that may have loosened the built-up rust/calcium enough to enable turning. You can repeat, just add more vinegar to the wrap. As soon as you can move the attachment enough to expose fresh threads, rinse off the vinegar, dry the threads and apply the WD-40.

When you replace the splitter, coat the threads with a bit of petroleum jelly- it really does help prevent build-up on metal threads.

Another thing to try is to run the hose until the water is cold and has chilled the faucet, and then run a blowtorch (or hair dryer or other heat source) over the splitter to warm it up. The cold water should cause the faucet to contract "a teeny smidge" and the heat should cause the splitter to expand "a teeny smidge" and then you may be able to muscle it off. BE SURE TO USE AN OVEN MITT -- the splitter will burn you!

It may be easier to get some leverage on it if you attach hoses to the splitter, if you have the smaller kind. Having the hoses on gives you a bit more of a hand-hold (just be careful not to melt your hoses if you use the blowtorch method!).

Hi I'm posting from the future! A decade after these posts, that is... Just telling anyone who's reading that VINEGAR IN A BAGGY TAPED AROUND THE NECK OF THE HOSE BIB WORKED. It was a miracle. After 45 minutes, I cut off the baggy and the splitter came off smooth as silk by simply turning left (lefty loosey) .