The discussion on negative keywords reminded me of a experience I had last year that is probably more common than I think.
I sell hand made wigits, and one of my wigit makers produced tiny samples that could be sent to prospective buyers. "If you would like to receive a free sample, please email us" appeared on the web page where I sell this wigit. I was getting two or three requests a month.
Then all of a sudden within two days I was deluged with requests for free samples from Canada! By corresponding with one of the free sample requesters I discovered that she was a member of a chat group whose sole purpose in life was accumulating free samples of whatever from wherever.
When I had difficulty tracking down this chat site, my correspondent gave me a link (it was part of another site having nothing to do with samples). She then sweetly mentioned that a major cosmetics manufacturer had encountered this same problem last year and in gratitude for her "help", had sent her a $10 coupon for more free cosmetics, and that I should do the same!
To me this is a form of blackmail. We are a small business and the free samples were intended for prospective buyers who might hesitate to buy the full size of a modestly expensive item. Needless to say, I removed the "free sample" option immediately (and she didn't get a $10 coupon, either). Since I believe they found my page by searching either for "free" or "free sample," I eliminated all references to anything free on the site.
The moral seems to be: Some people will go to any length to get a finger nail sized freebee. So be careful what you offer for free!