Add an option to the Mission Plugin API to enable/disable the ability to retreat, for a given FleetSide.
For the player, this should act the same as when they near the edge of the map and then immediately click "No"
For the A.I., it would be a little more complicated. The thing that seems to make the most sense to me is, if the AI would normally be about to retreat, but that ability has been disabled, it would instead make them suddenly and completely suicidal. Since retreat isn't an option, that's really all it leaves; try to do as much damage as possible.