I'm sorry to hear about the unpleasant experiences that you had in other clubs/sites. I try to keep this clubs as friendly as possible. From your experiences here and on other sites, how can we do that? What should be avoided?
In this sense there is the banning system. When you register a letter, you can also add the sender to the banned list and avoid future interactions with that member. You can also block a member by canceling an exchange. You can view your blocked list in the left menu.
When I receive complains, I try as much as I can to solve the reported problems. In most cases there is understanding and cooperation. Unfotunately, recently I had a first case where I had to block an account from sending new letters until the current exchanges are completed and after that it will be closed.