I would simply write back and say:
Thank you for your note. Most newer players actually need the coins/supplies they get from returning gifts, no matter the original gift, but if you'd rather just use diamonds to buy those things, I'll try to comply with your wishes. A good way to avoid gifts is to never click to return them. Eventually, you drop off the list and are only "bothered" with gifts when players do a full sweep of all neighbors, which is less frequent. I hope that tip helps!
I have made a great many friends in our neighborhood, out of the 340 neighbors we share (or that you will share with me eventually). I'll make sure my friends know that you don't want extra coins/supplies. Most big players only gift to help the little guys anyway. We try to be nice. We have more coins/supplies/culture than we know what to do with and neighbor visits take tons of time, even when we fly through the list and don't stop to check every name, as you request. So you may still get unwanted gifts from us. I heartily apologize for that!
On a bright note, it's much easier to avoid names on the trader than it is on the gift list. I'm usually helping all the little guys with their trades there, so I may still take one of yours on occasion. But I'll do my best to comply with your wish not to have help. Thanks for letting me know. My best to you. I'm sure you'll need it in future!