Stop being a victim, just stop playing that role. You need to redefine the relationship roles you have accepted in your mind of them as the bullies and you as the victim to just 2 equal neighbours who don't like each other!
Them: "Not good, not good" You: "Oh just piss off" or "Yeah thanks, that's helpful" (eyeroll).
Them: "Your cat came in our cat flap again" You: "Yes, he's a cat. If you want a cat flap which only lets your own cats in, with special magnetic collars, then buy one. Otherwise, just shoo him out".
Try to mentally roll your eyes/say 'twat' under your breath when you feel they're bullying (not so they see/hear you, but to remind yourself you are not a victim, they're being a twat, so react accordingly).
You do have to pay for the car scratch though, obviously. But you don't have to grovel, just hand over the money and slam the door. You're not a child or a victim, don't let the make you feel like one, or act like one.