How to stop cat spraying indoors? Are indoor electric fences useful?

Personally, I've never found any benefit in using indoor electric fences to manage urine marking. Be sure that the cleaners that you've used to clean up previous urine spots are enzymatic cleaners (Nature's Miracle is one example). Urine marking/spraying typically occurs on vertical surfaces (such as walls or the sides of furniture), where as inappropriate urination occurs more on horizontal surfaces (the floor, beds, etc.). It is important to differentiate between the two since they are typically caused by different things. Urine marking/spraying occurs most commonly in intact male cats, or those neutered later in life, as a result of behavioral changes from hormonal influences. Inappropriate urination on horizontal surfaces can occur due to behavioral changes (typically stress-based) or medical problems (urinary tract infection, bladder stones, crystals in the urine, etc.). If your cat is an intact male, I recommend having him neutered. It is also a good idea to have a urine sample evaluated for signs of infection or crystals to rule out some medical problems that could be leading to this issue. Otherwise, add at least 1 extra litter box to your house, scoop the litter box 1-2 times per day, change the litter at least weekly, consider adding or removing a hood/cover to the box (depending upon the current set up), and use calming pheromone collars/diffusers (Feliway is one example). I hope this helps and that all goes well!