Yeo Valley - after an encounter with a really rude customer services staff member. I still mutter "fuck off" at their yoghurts in the supermarket now, and it must have been ten years now.
Car Shop - sold us a disaster of a car, insisted nothing was wrong with it, then denied ever selling it to us even though we had proof. In the end we had to get rid of it, but they still spent two years ringing us up to ask us to book in the car they "never sold us" with them for a service or MOT etc.
Eon - we weren't even their customer but they kept threatening us about an unpaid bill. Worse, they were writing to our house about a shop at the other side of town, owned by someone with a different name to us. But trying to resolve it got even more bizarre because they couldn't grasp that they had got the wrong people and wrong address, and kept asking for the meter reading for our house to rectify things. I kept saying "this bill is for a shop, and you are trying to send it to the shop owners home address - but you are sending it to my house by mistake. I am not your customer, I don't own the shop, the shop owner doesn't live at this address, my meter reading is irrelevant." In the end I had to get the ombudsman involved and threaten them with breaching Data Protection before they left me alone.
Solar Frame Windows and Doors - let us down on our order but were then really rude about us cancelling because they couldn't provide the service they had sold us, and even though they actually took our deposit payment over the phone because they couldn't stand to wait half an hour for their salesman to get back to our house, they told us they don't send a refund cheque until ten days after the cancellation has been accepted. This after saying they didn't need our money anyway. But they certainly tried to hang onto it for long enough all the same.