How can I prevent other users from opening mail?

We have several apple computers around the house. Each one off course has Mail software. But Apple Mail (with .mac account) is only used by me while all the other members of my family use aol, gmail. They can and do use (when they remember to do it that way) the internet to check their mail.
However, most of the time they inadvertently click on the Mail stamp icon as soon as they get the urge to write an e-mail which obviously opens the mailbox. I would like to know if there is any way to create a password requirement before one can go into Mail.