I've got a serious cat urine problem

[Aaron Mazzrillo]

I bought a property about 2 months ago from an elderly cat lady. I couldn't believe she was still living in the house - it smelled so bad - just burned the eyes. I've purchased lots of cat pee houses and even own one that was worse than this one. That one was so bad you needed a respirator just to go inside AND IT STILL STUNK! I gutted that property, put in new drywall all along the bottom 24" and rented it out. Been rented for years and never a complaint. 

This new house wasn't bad once we get all her crap out that she left behind. I thought airing it out for a few days and giving it a bath in Odorxit would do the trick. We even sanded the hardwood floors and varnished them, twice. The house still reeks. It is a raised foundation and all sand underneath. I smelled the air near one of the vents and the cats were definitely using the sand as a litter box. I made 1.5 gallons of Odorxit (from concentrate) and sprayed it under the house soaking the sand. The tenant said it still stinks. (Yes, I rented it out like this. I'm a jerk.) I went inside and sprayed Odorxit all along the baseboards and floor within 6" of the walls. The tenant said it still stinks. 

I did some research online and found this product BioKleen that I'm going to order and try. At this point, I'm thinking I need to remove all the drywall along the bottom 24" of the walls on this house too. It is just horrible. 

Any other suggestions? What are some of the things you've used to kill that horrible lingering ammonia smell?

(Disclaimer - when I say "I" I mean my contractor.)