What should I do?

I don't think there really is negative feedback here, as long as they used the code its on their side if they have activated it on their account. You are best off writing a support ticket or trying to contact these 2 people.

Why don't you ask these people? Most likely the keys were duplicated or you have made a typo when copy/pasting the keys, there is also a chance of them stealing the keys to activate on another account or selling it. In both cases you can "fix" the situation by providing new keys for the same games =\

By the way, both of them are real "leechers" and one is regifter, so their previous reputation makes me think of something real slim shady.

I guess negative feedback is nothing you should be really worried about (there are so many other things to bother you much more!), the most serious consequence here is not being able to enter a majority of SG Tools protected giveaways =p