공짜는 싫어요 is probably not an often used phrase, as people generally don't mind taking things for free. There is an old saying about how people love things that are given for free: 공짜라면 양잿물도 마신다. It might translate to "Some would even drink lye if it's free", meaning they are so blindly eager to receive free stuff. I would say 공짜는 싫어요 might be said when you are averse to receiving freebies because you know most such things are a cheap marketing decoy with little real value, or sometimes because you think of it as a debt in disguise. I can't however think of a common examples in real life.