I made the mistake a few years back of agreeing to set up a direct debit with a woman who called on behalf of Save the Children (I know, I know, but I'd just had DD, was really struggling and feeling emotional...). Very soon after that I was plagued with multiple phone calls from the same fundraising company to give to other charities. Requests to be taken off their contact list didn't work. The same request up the chain to whoever was the caller's supervisor at the time of the call didn't work. I accused them of harrassing me; didn't work. I'd patiently, and not so patiently explain, that the event that appeared to have triggered these deluge of unwanted calls was setting up the direct debit to Save the Children. I told them that if calls didn't stop, I would be cancelling that direct debit, so their company would be responsible for a charity losing out. The person I spoke to at that time did appear to take it seriously. However, the calls didn't stop, so I told the next caller that I was cancelling the direct debit. They tried to persuade me not to, but I explained that in my opinion this is the only way they would learn, as polite requests not be to harrassed didn't make any difference. After I had cancelled the direct debit, I called Save the Children and spoke to someone in fundraising. I told them what I had done and why. Said that agreeing to donate to them in the first place had turned out to be a huge mistake on my part. Told them that these fundraising tactics are unethical and aggressive.
The calls continued for a while and I actually took some pleasure in telling the caller my saga with Save the Children. I rarely get calls these days, but if I do I say that I disapprove on point of principle of this way of fundraising and so on point of principle I refuse to donate in this way.
I'm quite robust and generally given to stroppiness so this is all just an inconvenience and irritation to me. I dread to think what the experience would be like if I was vulnerable and easily persuaded. Horrifying.
It really feels that as far as charities are concerned, the end justifies any means.