If you really think gift cards are pointless, stop accepting them. Tell people it's a personal policy. They will respect that.

I don't know how you figure it isn't wasted money. Not only did you not receive the good or service that they spent their hard-earned money on - which is fine, you don't have to use it for yourself - you didn't even pass on the gift card and receive some smaller benefit. That's thoughtless and disrespectful - not the act of someone giving you a gift card, but you accepting and throwing it out.