Wow - so many things you could say!
Maybe something like "When the Witnesses start behaving like Jesus, I'll give you my time for the month."
Another reply that comes to mind is "Put me down for 60 hours. I've done a lot of "witnessing" online!" Keep telling them to put you down for 60 hours. They will eventually stop asking.
My personal experience was that I stopped field service and stopped turning in my time. I would get calls, texts, emails. I would delay the replies more and more until I finally ignored them.