It's worth noting that the goons are private contractors, and have absolutely no rights beyond what the average bloke down the street would have. They can't enter your house without permission, and they can't force you to sign documents.
In the UK, we have a legal theory called the implied invitation, which means that it is implied (because it would otherwise be ridiculous) that any member of the public is invited to walk on your property up to your front door. This allows the postal service to put letters through your letter box, and people to knock on your door to say hi. However, this right can be revoked. You can write to Capita and state that you are withdrawing the implied invitation to all its members and agents, and that they are not welcome on your property. Then, they are not allowed to enter your property and are trespassing if they do, and you can call the police straight away without even warning them (because you did warn them already by post). Capita (despite its many faults) does seem to obey this.
I did this for my property shortly after I moved in. A little while later, a group of people did wander down the road. Half of them came onto the property and knocked on the door, claimed to be from Sky, and asked if we wanted to get satellite TV. The other half were wearing a different uniform and stayed outside the property. I strongly suspect that half was Capita agents who asked some Sky employees to help them out. Unfortunately, it was my wife who answered. Although she told them to go away, I would have been angry at them, because they had made the Sky employees into Capita agents, and therefore they were trespassing.
The threatening letters are atrocious. They are scary for anyone who doesn't know what they are doing, as they almost imply that your door is going to be broken down and you'll be hauled off to prison. I called Capita out on this. I wrote to them declaring that any further threatening letters from them would be classed as harassment, and that the police would be involved. They stopped sending them.