Cold callers have long been torturing homeowners through a variety of tactics, usually through silent or highly vocal #telephone calls or door to door calls. The #legal rumours are if you have a sign prominently displayed requesting no visits then it could be a criminal offence; in theory at least according to an #anonymous QC. It's unhelpful that there hasn't been a reported benchmark ruling and so the law is largely toothless and ignored by companies and #organisations as there are no specific lines, or if there were perhaps the said parties wouldn't care.
On the nuisance telephone call front a #broadband provider was fined £750,000 for making over 9,000 silent calls. This may make them think twice or perhaps not! Presumably there are two main reasons companies persist with cold calls- either, it makes them too large a sum of money to give up and adequately covers any fines and slaps on the wrists they may receive, or because it's just so cheap to do and the chances of getting custom that way (however slim) outweighs the costs of being caught and punished.
At the front door you have a slightly different ball game when you’re interrupted face-to-face. It is arguably the more invasive of the two but that isn't to say that cold phone calls aren't annoying.