When they won't take no for an answer
My mum says she won't vote any more because she cannot understand who is who, and what their policies are, and therefore could not make an informed choice. I think this is very responsible of her. She brought me up to value my vote so much and takes it very seriously. Women died to win us the vote etc. etc.
So when she got a form last year asking her to re-register for the electoral roll we threw it away. I also rang the authorities to explain why she didn't wish to vote. This was followed by ever more insistent letters, then a threat to send someone round to the house to take the details, so in the end I filled in the form for a postal vote. When the ballot papers arrive I check with her that she still doesn't want to vote, then dispose of them. Personally I don't think it is right to complete the ballot paper on behalf of someone else unless the problem for them is purely a physical disability.
Perhaps the canvassers and nosy neighbours can be put off with you telling them that you've already done a postal vote and that as it is a secret ballot you don't want to discuss your choice/s?