Can this be done? Is there a Mod?

[mikeyb987]

I just started my forums chambanatalk.com [nofollow] It's a forum for the towns of Champaign- Urbana Illinois.  (University of Illinois) I also run one of the busiest restaurants on the campus.  I'm already giving out 4-$15 gift certificates from my restaurant for people to register.  At the end of the 2 week period I will randomly choose 4 people to win that registered.  I was just thinking about this and I realized that if someone registered you wouldn't want to tell other people because that would lower your chances of winning.  Dumb of me- I know  
Here's my new Idea-
I sell smoothies also, so I want to either give free smoothies away or sell them for a buck.  I'm sure I could make a coupon or something for people to print out once they register in a registered members only forum!  But I don't want people to be able to just keep printing them out and have 20 free smoothies.  Is there a way to send them a coupon only once and insure that they could only print it once?  I'm sure the printing idea would never work because as long as they keep the page open they could keep printing them out?  I guess I could number them and put the user name on them and keep a list at the restaurant for my cashier. Any ideas would be great.

[jerm]

i would suggest just to number them.. once that number has gotten their item, they are checked off the list.