You can't choose the winners but you are able to hold at least the worst out (with a lvl 1 or 2 restriction).
You don't hold out the cv farmers with it but at least anyone that don't shared SOMETHING with the users.

Groups, Whitelist and such stuff helps too but close out more and more people, so i don't adviced it to you.

I by myself gave, after 10 or 11 months of tries, mostly up to make public GA's. It go much too often to rulebreakers (with partly up to 9 entries of multiwins or not activated stuff -and no ban only a "he have served all infringements"-), at least for my taste.
I had GA's with 330 Entries and after the GA i checked them all and 180 of them landed on my BL. Such GA's make me no fun, so i do them anymore.
I switched to whitelist, private and groups because i am forced to do so, not because i want :o(
But cg/sg don't give you the tools on the hand to use the sgtools rules at public GA's, to hold at least the people out that have multiwins/not activated wins, and that is really bad. No tools to say each one that gave the $1 cv farming bundles 10+ away are closed out. Nothing.
You have only the level restriction as possibility.
Nearly all Staff Members use groups and or high levels to secure there GAs -i think that should say something too...-

You use a very nice Avatar. Did you know from where it is ?