Ehh...No idea. You could always use OnAsteroidTaken() to reset it when it is captured.
Something like this:
function OnAsteroidTaken(id, owner)
if id = 5 and owner ~= 1 then
That (I think) would set the Gracetime of Asteroid 5 to 600 seconds, unless the conquerer was the player. Whether the conquered AI will then start attacking it again, I don't know.
Globals.AI.GraceTimer=(9999999999999999999) would set the GraceTime of EVERY AI to such a ridiculously high number as to be infinite, and stop the game playing the AI at all. Math.huge is the Lua equivalent of infinity.