@TobysPaw,
I think there is not really a solution.
The best I can think of is to reduce social interaction so the contact falls below other contacts in your chat, and will stop seeing you there. Deactivating the chat just turns the green "online" poiint always to grey, but still shows your timestamp. There is an extension for Google Chrome called "Facebook Unseen", which at least avoids that the contact can see when or if you read his messages.
I have a question regarding facebook too, maybe you know the answer?
A contact disappeared from my FB Messenger App list, this happens when the contact blocks me. But I can still see his timestamp with the computer in the chat bar, which can not happen if I am blocked. The contact is still using FB messenger app - Does anyone know what happened here?