CrossRoads:
When it senses crossing one, note that, and stop when the second was crossed.
This will only work if you can be certain that any bot reaching the black line will recognise it, stop and back up without actually crossing the line. I'm not sure that is feasible, given that a robot might be trying to back up into the arena but actually end up being pushed out of it. The only scheme I can see that seems reliable is to have a greyscale or colour gradient at the boundary of the fighting area and have each 'bot try to head back towards the lighter area - then have a black boundary and have the 'bots die if they find themselves on it.