[newbie question] Any tips to make giveaway only be winnable by interested parties?

Make a steam group, invite whoever you want - make a group-only GA for that group.

Add interested people to your whitelist - make a whitelist only GA.

make a whitelist giveaway, invite only people interested in the game you offer.

recruit them with a filter, like, have "this" game wishlisted before X date.

• Blacklist people you dont want to join your giveaways
  or
• Private giveaway, give link just to people you want.
  or
• Whitelist giveaway
  or all 3.

You can micromanage who can join by using group or private giveaways and have people apply through a Discussions thread for instance.
If your rule can be phrased in a general measure, you can use the SGTools software to automatically apply it to your private giveaways.

i d suggest an approach that will be a little bit different but still you dont have to check everyone that enters by hand.
make a riddle/puzzle with http://www.itstoohard.com/create and just type a few questions about said game and whatever you come up with :)
if someone is interested they will solve it and join. bots are pretty much excluded that way. yes.. anyone who solves it can too.. but cmon... who goes through the trouble to solve a puzzle... and if... didnt they earn a chance?
oh and users who prefer private profiles can also join ;)