Maybe they have a quest to kill x amount of minions and they can go ahead and kill a few more of yours. Maybe they need to summon so many minions. Does it really hurt you to wait 10 more seconds? They outplayed you (or got lucky, right?), allow them their victory. If they start healing and doing stupid stuff like that just to rub it in, concede. If they're just killing minions and playing cards for more xp, let them.