Place units near to where walls may be formed to stop opponents walls from forming. (Remeber it is generally considered unsportsmanlike to enter opponent territory during turn 1).
Remove the old wall and replace with a new wall during the same turn forcing your opponent to use 2 units, to break the wall and prevent the new wall being made. Be careful the new wall does not get turn-blocked.
Remove the old wall and replace with a new wall during the same turn forcing your opponent to use 2 units, to break the wall and prevent the new wall being made. Be careful the new wall does not get turn-blocked.