I would suggest the best deterrent for this enemy cat is you.
Im am extremely anti-cat-social when im around my home (outside) I'm mean and hissy and a stomping loud meanie ... before I even started to let my girl out I got her used to a few unpleasant noises (bonking a pan with wooden spoon) to the point where they didnt make her jump or anything.... and then I took those noises outside to make neighbour cats wary of my garden and that mean human who clatters and bangs around.
My garden is totally open (for a cat anyway) but these days I never see neighbour cats come around .. they are avoiding me and by proxy avoiding my girls territory.
It doesnt feel nice to be mean and loud and chase away other cats but tbh the chances of them ever becoming friendly and a companion to my own cat are SO slim - I decided its better they hate me and stay away from me AND my girl's garden.
Later down the line when I began walking Cleo on streets and around my neighbourhood we would often encounter other cats (still do ofc) and the situation would be -- other cat sees ME and thinks.. ''god not that horrible noisy human, Im staying clear of him'' -- my cat see this reaction and thinks ''gosh that enemy cat is scared of me and keeping distance, so I dont need to worry then''
I have actually met ONE nice cat (possible friend) that Cleo and I have bumped into a few times and she is just SO lovely and keen to be friendly to Cleo - displaying the most respectful cat language to try to make friends with my girl... thats the only cat im nice to... when we see her (not often) I keep Cleo calm and pop treats under her nose from my pocket and let this sweet friendly cat come close upto us .. as yet Cleo hasnt let her sniff or approach close to her yet... but maybe one day