Finish one more UFO, and then you can reward yourself with new yarn for the project. I've even ordered the yarn online, then forced myself to finish a UFO (must be a good-sized UFO to warrant the good-sized reward) before the new yarn arrives. Don't make the self-imposed deadline?? Then don't let yourself open the package until you ARE finished.
This way your stash won't actually "grow" with the new purchase, because you also took some out. And (bonus!) you'll be a project ahead! Good luck on whatever you decide!