Stray cat terrorizing my cats! Curious about solutions to bother enemy cat!?

There is a cat that shows up in front of my screen door and terrorizes my older cats. I am afraid my older cats (who are indoor) will have a heart attack or suffer serious health problems from such a traumatizing experience. I want to know what type of liquid solutions (in addition to water) really bother but don't seriously harm cats so that I can scare this cat away for good.