Perhaps it doesn't work for everybody, but in my case I keep challenging my mother with facts. My mother is exactly this type as well, perhaps not as combative.
One of the last times she tried to preach to me, I simply told her that according to the Org, the world governments will shut down organized religion and then eventually come after Jdubs. I told her that for this to happen we will need to see freedom of religion be abolished, change in constitutions in many different countries. And that once that happens, at least we'll know the Org will be right about something and I'll consider coming back. Now, of course none of that will happen.
So now, when she tries to preach I simply point to that. That I'm still waiting for this to happen, and this has been effective in pushing back and shutting it down. In her mind, she probably reluctantly admits that those things do need to happen first and they are not happening.