Also when they are stationed in shops/ petrol stations that you want to use so you have to get by them.
It baffles me as to why shops agree to this. I don't know if they think it makes them look kind and community-minded, or if they just get browbeaten by the charities refusing to take No for an answer. I don't mind somebody standing to the side, waiting to be approached by interested people; but that's never how it happens. Somebody will rattle a tin under your nose and say something like "Help children with cancer?" and know that most people will feel to ashamed to ignore them.
Our local Co-op used to have a different charity every week - sometimes they would approach me whilst I was clearly actively shopping (looking at goods and comparing prices). It really put me off using that shop and I would often go elsewhere in preference.
I also hate it when you get local groups - Guides, Scouts or whatever - who immediately start packing your bags without asking, clearly for a donation. I'm fussy about how I pack and I don't want a child throwing tins on top of eggs or salad. I don't object to supporting them with a quid (still feels a bit pressured, though), but I'll pack my own shopping, thanks!
Best suggestion I can think of, once they've deliberately ignored your declension, is to sing 2 Unlimited at them loudly, without stopping. "No, no, no, no, no, no, no, no, no, no, no, no, no, no thank you!" That song is irritating enough just as a song, so hopefully they'll get the message. If not, ramp up the volume with a "Techno, techno, techno nonsense!" If nothing else, they'll assume you're crazy and steer well clear 
Somebody should develop a simple website - something like CharityCollectorDueDilligenceCheck.com - with questions that you can require them to complete before you will look at their form:
What is the charity number?
How long have they been working for the charity?
How much do they personally give to the charity each month?
What first attracted them to the charity and its causes?
Can you guarantee that any money I give/agree to give today will be handed straight to the charity, without any commission?
If no, and they are expecting to take commission, why wouldn't I be better giving directly to the charity, so that they get all of it directly?
Note: I will not accept 'we take our commission out of the gift aid, so all of your giving goes straight to the charity' - as that gift aid is also my taxes that I can choose to reallocate to the charity on top of my giving, so that they get the maximum amount.
If they won't fill in your form, how could they possibly expect you to fill in theirs? Get THEM having to keep saying No (or just go away, preferably).