Since this appears to create new forces (biter_faction_x) its impossible to use the provided lua function on the wiki to deal with the "enemy" force.
Is there some easy way to iterate over all forces that have your new name and execute the normal functions like?
/c game.forces["enemy"].kill_all_units()
The reply by JE3 is good enough, iterating through all forces in the game list may accidentally kill your own stuff or your friends if using other faction/team names.