I highly recommend the Feliway plug in diffusers. They are terrific, not only with territorial issues, but just in general helping to keep the kitties feeling secure and content.
Just make sure you have enough plugged in. Each one is good for about 400 sq feet, so if you have a multilevel house you should ideally have at least one on each floor.
Also a tip to make them last longer: Save the cap to the vial, and, if you have multiple plug ins, start a rotation where you unplug one at a time for a week (cap it to keep it from drying out). So, you always have at least one or two going, but, alternating, can keep the pheromones circulating, while it lasts longer.