Orange peel, grapefruit peel or lemon peel work best if you keep them fresh (every 4 days or so, they really hate the citrus oils). Limes & tangerine don't have the higher concentration of oils in the peel so they don't work as well.
If it's stress related, "Feliway" is a great cat pheromone to calm down anxious cats but check around for pricing. I was looking at the 75ml size bottle online which was $18 and the pet store had it for $35! A little goes a very long way. The liquid spray is better value than the diffuser.
Otherwise I've used a cleaner called 'dogtergent' that takes all the smell out of a carpet if the pet went the wrong place, because vinegar or soap and water just won't remove all the odors.
Is it a full bladder's worth each time? Or is it little dribbles? If it's little bits, you'll have to check for a bladder infection, cats who have those can't help where they go and use places outside the litter box because they get frustrated that they can't relieve the problem with the box. The only way to stop that kind of problem will be with antibiotics. It's not the cat's fault in this case.