Sage103082: Bots can be used in the forums to collect codes? That is just wrong on so many levels. Like Ragnarblackmane, I have to keep the curse words from coming from my mouth to keyboard... sick f......
Yes. Anyone with some basic scripting competence can cook something up that scans the GOG forums for links and codes, and enters them at their respective redemption sites. It's not particularly difficult, especially if it all takes place in a browser.
The best way to addressing this, if you care about it, is MAHAYO ("Make A Human Answer YOu"), i.e. give your codes away in a way that only a real human can use them. Techniques include leaving out a few characters and posting them further below, posting a riddle that leads to the code, or sending your code only after a simple task has been performed ("post your favorite ice cream flavor", "write a seven word SciFi story", etc.).
IndieGala has an interesting feature, it shows you to which Steam account exactly a key was redeemed. I'm not sure if Humble provides this feature, though. And in any case, the OP may notz be interested, after all his intention was to make some people happy (which he did), not to spend time hunting possible leeches.