Hi R. -
Here's a response I posted to a similar question...
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Hey Jennifer --
Been there done that...
1) Go buy a gallon size of Nature's Miracle. Be prepared to buy more.
2)You don't say if your big boy is neutered. If not, make the appointment.
3) Buy at least one more cat box. Keep ALL of the catboxes immaculately clean (at least for now). Clean them once a day. Once a week, take them outside and hose them down. Introduce big boy to the new cat box -- this one will now be his.\
4) Go around the house and thoroughly spray each area the cats have peed/pooped on with Nature's Miracle.
5) Rent a rug cleaner, add about 1 cup of Nature's Miracle to the solution before you begin to clean. Clean all of the carpets thoroughly
6) Mix 1 cup Nature's Miracle in a bucket of cleaning water (with Spic N Span, or something). With a sponge, go through the house and clean up wherever pee might have gotten -- floors, walls, etc.
7) If there is a specific place where the cats are peeing, cover it with tin foil (beds, couches, carpets). Cats HATE walking on foil. Crumple it up first, then straighten it out -- Tack it down so that the cats can't move it.
8) To the extent possible, close off the rooms where the cats have been peeing. WATCH THEM LIKE A HAWK. If they even enter into a room where they have been bad, bark at them!
9) When you see your cats use the litterbox, give them a treat, strokes, and happy words.
Keep at it -- it's a hard habit to break, but you CAN do it. Just like potty training a toddler, lol!
Best,
J.