commit
to a breaking strategy (as either alignment, although more commonly I see this done from scum). If the strategy is sufficient to win the game, or if (due to incorrect assumptions) it automatically loses the game, call the game right there; otherwise, continue the game. (Mathematically speaking, to avoid abuse, you might have to modkill players who deviated from stated aspects of the strategy. In practice this is unlikely to be a problem, as the information that the strategy neither automatically wins nor automatically loses is not likely to be helpful in most cases.)In the case of a breaking strategy for town, I see no issue with simply allowing scum to surrender if they want to (as they can no longer win, all actions are thus playing to their win condition). I guess it's theoretically possible that the scum could talk the town out of it, and thus they should be given a chance to do so if they wish.
As for automatically calling the game (i.e. unilaterally by the mod, with no strategy commitments or surrenders by the player), that should happen when the result will be inevitable on any normal play. For example, if merely attempting to nightkill every night is sufficient for a scum win, scum should probably just be given the win immediately. Automatically calling the game as a town win should (with normal mechanics) only happen when all the players left alive are townies.
Note that in Opens, you can normally write win conditions to avoid the issue arising in the first place. (This isn't always possible in Closed games as doing so may potentially give away too much information about the setup.)