You need a game_round_win or whatever that entity is. Set it to no team (sudden death).
Make your game timer, pressuming you have one, point to the game_round_win when "OnFinished", with the command "roundwin."
If you want to take it a step further, make a point_template for each of the flags, name the flags, and make each point template have a template for its respective flag (ex blu_template with a template1 of bluflag and a red_template with a template1 of redflag). Make sure all flags are checked for the point_template. When the timer is OnFinished, make it point to each flag with the "kill" command, then with a delay of one second, point to the templates with a command of ForceSpawn.
So to recap, timer outputs:
OnFinished suddendeathwinthingy roundwin (no delay)
OnFinished redflag kill (no delay)
onfinished bluflag kill (no delay)
onfinished blutemplate forcespawn (1 second delay)
onfinished redtemplate forcespawn (1 second delay)
Templates will be set up as follows:
name: blutemplate
template1: bluflag
template 2/3/4/etc: none
both flags checked
Copy the template for red, but with "redtemplate" and "redflag" instead of blutemplate/bluflag.