We rehomed a 12 year old cat 6 months ago and he was obese. So our first job as specified by the rehoming charity was to put him on a vet directed diet to get him from 6.2kg to his target weight of 5kg, which we are trying to do. We got him down to 5.9kg and now he's gone up to 6.3kg.
It turns out he would go into neighbours' houses and steal their cats' food so we've been round and apologised for this. We have repeatedly asked neighbours not to feed him, particular NDN. We went round to NDN again last week and she said she had just given him his lunch. 
So we have asked her again (now for the 4th time) not to feed him as it will harm his health. Today he didn't eat all of his dinner so I know he's been fed again. Hence my AIBU because surely if she's still feeding him and she knows it's bad for him, she's deliberately harming him?
He's going into the vets to have a couple of teeth out next week and I don't know how to stop him eating elsewhere, especially while he has stiches. He has long hair so a collar saying DO NOT FEED ME won't be very visible. I'm guessing the only option is to keep him inside, but not sure how to address the feeding long term. He's just getting fatter and fatter and we keep being told off by the vets who just says we have to tell the neighbours not to feed him.