I would wager that, if you get a large ant mill going, the level of pheromone signal would become enormous and be sustained for as long as the mill is going and the ants are alive. So, a sustained, very high level of pheromones might trigger some kind of break-glass response where they invert their logic and seek _low_ levels of pheromone, seek if get _off_ of their track. Or perhaps they begin to erase the pheromone trail (if they can).

Which breaks the ant mill, but now you have to get back to having a functioning colony. Don't know how that would work.