You cannot create additional rules apart from the built in ones (groups, whitelist/blacklist, level restriction) to filter out the people that join the giveaway, sorry. You can actually get banned for what you're trying to do here.

Edit: I've reported you both here and on SteamRep and they asked to let you know of the rules in case you aren't aware of them:
http://forums.steamrep.com/threads/what-constitutes-reputation.12391/

Thanks!

Thanks!

Thank you

Thanks

Thank you!

Thanks

Thank you! :)

Thanks

Thanks :)

Thank you.

rep