Just ignore it and let the debt collection agency write a few more letters (they won't come to your door, they just keep threatening it as their business model is built on intimidation). You are not in any way obliged to engage with them. Eventually they will just give up. It would be nice to see some of the less admirable members of our species waste some more of their time, money and energy.
All joking aside, the hospital have a data protection breach on their hands. I would return the letter with a note to explain the error and forget about it. Happy Christmas.