How do you deter hostile outdoor cat?

[Greenport ferals]

I've been through this many times with a stray tom. Trap, neuter, release. Otherwise you're going to have a vet bill, or a series of them, when he fights with your cat. It's cheaper in the long run to just get him fixed, and will benefit him too.
Call your local rescue or humane group about borrowing a trap and details about low-cost neuter options.

[Straysmommy]

I wouldn't mind if he's owned or not. I would TNA him (Trap-Neuter-Adopt). I did this with the bully stray downstairs. He's still the same bully, but neutered cats aren't dangerous. He still lives outside because I can't adopt all my strays (I already adopted 3 of them) but he's more pampered by me than many house cats are by their owners. Prince and Nikita, when they come downstairs with me, are still afraid of him, but he's far from the menace he used to be. And last night, after 2 years of keeping a good distance from me, he suddenly came up to me and started rubbing against my legs for a long time, a huge surprise for me.