If you only offered 2 game challenges to people rated higher than you, this is a pretty good way of legally increasing your rating without having to play chess at all.
That said, the way to avoid it, is not to accept any two game matches like this. Offer instead to start the 2nd game a few moves after the 1st.
To answer your question of what to do after this already is happening, I think there is no solution. You gave some good ideas, but like you said, nothing that guarantees you'll be able to stop it.
[COMMENT DELETED]