I used to work for one. They rely on repeat donors mostly as new donors give tiny amounts and vanish.
Charities ask for increases, most ignore the ask, a core group don't. Over time that core group are more likely to give bigger donations and leave legacies too - they're often the ones making things work consistently, despite the occasional amazing "crowdfunding" effect like the ice bucket challenge. It's probably the same for the Guardian.
When you're utterly passionate about a cause you often don't mind being asked, even if you can't give more. (Some people do undoubtedly but many don't, or don't mind enough to stop their existing donation.)
When you aren't that zealous about the cause, or you can't afford it, or just really don't like the idea of being asked for more - well, the charity just doesn't know that without asking you. The bigger more organised ones will have databases and people to write stuff in them to avoid asking you again once you've told them not to.
It's a broad numbers game and having seen the immense good that charities do with their funding, I still "get" it. It's rather unBritish though! Some charities may be worse than others of course, I can't speak for all of them or their practices, and perhaps some of them do pester too much.
But legally they should leave you alone Morris; chances are they might have three or four records for you - or you've unsubscribed from one appeal and they're still talking about others, and don't know you're annoyed by it.
You could email them again clearly with the details you want removed, and say you want no further contact from them at all ever, or you'll pursue it further with the charity commissioner?
sorry for the long post