The robber keeps moving
The robber may not be returned to the hex it was just moved from. Whoever rolls a 7 must find it a new home. Stops two players from locking the same wheat hex out of the game all evening.
What to know before you play it
The problem this solves is specific: two players discover that the same ore or wheat hex is everyone else's lifeline, and take turns putting the robber back on it. Nobody is cheating, and the game stops anyway.
One line settles it, and it is easy to remember because it needs no bookkeeping — you only have to know where the robber just was. Pair it with the friendly robber if you like; they do not interact.
Worth knowing before you adopt it: on a board where one number is genuinely dominant, forcing the robber to move around can make that hex better than it should be, because it can no longer be pinned. If your group's complaint is a runaway leader rather than a stalemate, this is not the rule you want.
Does your table play this?