I wasn't suggesting that guards instantly kill them, but perhaps aggro them on sight, or even archers like you suggested. In any case, something to deter those playing these races from doing this.

Maybe arrow fire when they are one hex away, but they get aggro'd if they try to enter the city.

Just a suggestion based on a few observations... Guards or patrols of guards should be set up on the roads near civilized areas. That either drive back the "monster races" or try to kill them as per cannon. It seems that everytime someone playing a monster race (half-drow, tieflings, orcs)...