Spay or neuter the cat
Clean all sprayed areas with an enzymatic cleaner
Identify and remove stress triggers
Provide enough litter boxes
Keep litter boxes clean
Use unscented litter
Place litter boxes in quiet, accessible locations
Separate cats that are in conflict
Add vertical space, hiding spots, and enrichment
Feed and play on a consistent schedule
Block access to previously sprayed areas
Use pheromone diffusers
Reduce outdoor cat visibility
Check for medical issues with a veterinarian
Consult a veterinary behaviorist if the spraying continues