Here's what you could do if you really wanted to mess with them a little..
You say the letter was in your door, not mailed, right?
Go down to the police station and file a report of harassment. Advise them of the situation, that you stopped going however long ago and don't consider yourself a member. The police will at least go have a talk with each of these fellas, might be fun if they squirm a bit.
The cops could also tell you if a restraining order is possible based on this, wouldn't it be lovely to see each of them served with a restraining order?