Well, there are different kinds of bots.

There are bots that just scrape websites for Steam keys and automatically try to redeem any they find, such that any key posted publicly on SG or Reddit or whatever is likely to be detected by web scraping bots within minutes. Thus if you want to post a key publicly (instead of just making an SG giveaway) and want a human to get it, you need to make it complicated such that a human can figure it out but a bot can't. For example:
ADFG8-FHT*4-TH3%6

• is the first letter of the game my avatar is from
  % is the third letter of the game my Steam background is from
  (for illustrative purposes only, don't waste your time trying to solve it)

This way, a human can figure that out without much effort (particularly for your avatar and background), but it's too complex for a bot to do.

Then there are bots that scrape SG giveaways and automatically enter giveaways. I'm not really sure what you can do about those other than using something like SGTools to protect the giveaway or increasing the level of your giveaway. Most of those bots are level 0, with few at level 1, and very few at higher levels, so doing a higher level giveaway, even level 2, would cut the vast majority of them out.