I had the same problem, with some one feeding one of my cats, I put a collar on her and wrote on it in marker pen. Needs Medication daily - Do not feed. (bright yellow collar with black marker pen, couldnt miss it)
It worked, she stopped feeding her althought they did ask if they could heve some tablets to keep at there house, in case she was there.
Told them shes on injections.
I would advise to give the collar ago. worth a try.