If you get someone that doesn't want to let you use the coupon bring up this website and inform them you will just go to another Target.
Edit: Obviously YMMV, but that works more often than you'd think.
Just went and picked up 3 pre order cards and got the coupon on the first transaction. Told him i didn't need the other two and walked out. Gonna try again tomorrow when I go to pick up my PS4 games.