Airtricty is sold door to door (and I assume in the lobbies of Tescos) by commission only sales people (I believe their company is PMD Marketing but I may be wrong on this).
They have no information as to who has been 'switched' previously which is why, if you live in what is deemed to be a receptive area, you will continually find them at your door.
My advice for dealing with those in the lobbies of Tescos is just to say 'no thank you' and walk on, ignoring anything else they have to say. At the end of the day they have a job to do and they actually are trying to save you money against certain market incumbents.
My advice for dealing with cold callers to the door is to listen to their pitch and take on board the figures they are quoting you. If you know they aren't any cheaper than your current supplier (be it gas, electric, phone, broadband, bins...) tell them this and generally they will go away (there's no point in them wasting their time if you're not going to sign). If on the other hand, you think their deal might be better, take their contact info and tell them you'll check it out... and do check it out, either online or by phoning the company yourself. If you do then want to switch get that salesperson back so they get the commission. After all... everybody could do with saving a bob or two at the moment and these guys are just trying to earn a few bob.