Greetings: 

The Environmental Protection Agency (EPA) has registered over thirty compounds for safe use in repelling domestic cats. Check with any pet supply store or garden suppliers for commercial cat repellents.

Also, 

Inappropriate Elimination: Cats change their elimination areas to other parts of the house. The problem may involve both urine and feces. This may be caused by a variety of causes. -medical problems-any disease causing increased thirst and urination can cause this apparent behavior. Such diseases include diabetes, kidney disease, liver disease, bladder infections, bladder stones, pancreatic disease etc. Geriatric cats with arthritis, thyroid disease, cardiac disease may have problems accessing a litter box.

-litter box aversion- dirty litter boxes, changes in types or odors of the litter box

-anxiety

Treatment:

-complete medical work up to rule out pre- existing diseases

-clean soiled areas with enzymatic cleaners

-limit access to soiled areas by confinement

-make the litter box more attractive –offer different types of litter , multiple boxes

-make the soiled area more aversive

-if inappropriate elimination is in a specific location, place the box in those locations until the cat is using the box and move it gradually back to a more appropriate location

Another suggestion from the Rocky Mountain Cat Alliance:

The Mystic Wave