Just as some people can't withstand crowds, some cats also seem to get stressed when other cats are about. Many times sprayers DO change their ways when introduced into another environment, so re-homing may be successful, especially if no other cats are in the new domain.

Replacing cinder blocks due to the smell??? Sounds like an extreme measure, so must have been intact males because I notice no odor from my neutered cats' urine, and I have a female that has sprayed outside the litter box on many occassions. If it wasn't for the fact that I SAW her do it (and other times noticed a wet area) my keenly sensitive nose would not have detected it. Simple Solution sprayed on the area works great, and I only do it to eliminate any odor that probably the other cats could detect. I have even asked visitors to level with me if they notice any offensive odor, and I have been assured not.

Jen, I hope your kitties behave themselves soon, and appreciate all the love and care they are fortunate enough to receive from you.