How to stop a specific whatsapp chat from using mobile data?
It is a good requirement but unfortunately Whatsapp didn't have such a settings.

If you want to avoid getting notifications, you can Mute the group.

But here, your requirement is different.

So, First understand how the blocking works.


It works by the way we are blocking any access to the particular host from our phone. That means, no request, no response.
Here, whatsapp use same host for all messages, you can't block it.


If whatsapp use different host for different chat , there is no problem. But here it is not the case.

So, either you can block entire whatsapp or you can't block anything at all