Even though I completely understand your aversion to getting the plants, I'm a big believer that a gift is purely a gift and should be accepted at face value with pleasure for both parties involved. After you've received the gift, it's served its purpose of being a gift, and you are free to do as you please with it.

I have a green thumb, but don't care to have people gift plants for a couple of reasons- some aren't cat safe, space is at a premium, a strong preference for certain varieties. People get the wrong idea that I love plants and that anything they bring is a welcome addition. Here's what I do though. Just be grateful, as though someone brought you flowers (that won't be around long). Then... 1) if you like it, give it a place in your home and see if it thrives on benign neglect. 2) if possible place it in an outside area and see if it likes living in your climate (I live in an unforgiving climate and am surprised at how many do well outside) 3) regift it to another friend or someone else who might enjoy it. 4) after the friend is gone, place it in a common area with a free sign.

If the friend comes back to visit and asks about the plant, tell them that you don't do well with many plants. Maybe cacti and succulents are the exception. They'll get the hint without being offended or hurt. They may also notice, not say anything, but stop bringing you plants.