Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear about sweet Nikki's condition...I know this is frustrating.
From what you have told me, it appears that Nikki is suffering from some type of stress that has caused this inappropriate elimination. This could be due to a territorial issue or some other stress. This is not an uncommon problem, especially in the pure breeds. The first thing to do is to have your vet examine Nikki to be sure there is not a medical reason for this behavior. If this is ruled out, the next thing you need to do is to purchase from any pet store a product called Feliway. This is a feline pheromone that acts as a calming agent and will discourage this bad behavior. Follow the directions on the bottle. Next, make sure to change all litter boxes to different places especially where there is little traffic. Keep these boxes extremely clean.
Try using different types of litter and make sure to clean these areas with an odor eliminator such as Nature’s Miracle.
Use aluminum foil or double sided tape in the problem areas which may discourage using these spots.
If this is not sufficient, your vet has medication that can help with this problem and, if used along with the Feliway, hopefully, will be successful. These medications are antianxiety drugs such as Fluoxetine or Clomipramine. Here is a website that has more tips:
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****