This is not the first time I have mentioned the Charities Aid Foundation cafonline.org - hope the link works as I am wearing my technically-incompetent hat as usual. If you set up an account with anything from £10 per month upwards, they claim back the Gift Aid (if you are eligible) and add it to your account, and take a small contribution from it each month. Then you can send regular or one-off payments to your chosen charities and if you choose to do it anonymously, they don't know who you are and can't get at you with Christmas cards or anything else.
As for these 'free gifts', which still come from charities you may never want to support, I too just either use them (pens and notebooks) or send them back (cards). It has put me off supporting the Red Cross, although we haven't had anything from them for a while, so maybe they have rethought. The Cats' Protection League, on the other hand...