instead of ending the game, you could make the forfeit button available *ONLY* for cases where the other is dominating.
so if a player owns, say, 70% of all troops, the forfeit button becomes available, and only if both players agree then the game ends with a winner.
although that would be problematic in fog of war games, if you're really not sure how well everybody is doing, and one player has been stacking for lots of turns.