Been through this just recently. It drove me crazy but several of the boys decided to start spraying and marking all over the house! I thought I was going to go nuts! I finally figured out what was making mine do it. On pretty days we open up our windows and it turns out that the ferals I was feeding were, of course, spraying everywhere outside to mark territory. My boys felt the need to mark theirs inside! Now that the windows are closed I had a hard time breaking the habit. Don't ask me how I did it because I really don't know. Everytime I saw or smelled a spot I sprayed it with Urine and Stain remover and I also yelled, sprayed, and stomped everytime I saw one of them backing into something. I think I finally won....for now that is.

Can you move the litter box to that corner temporarily? At least for a lttle bit until they move on to the next bad thing they are going to do and forget about that corner!