Browser back page button reenters private area after log out

Hi, I’m implementing a site where wordpress and Customer Area plug in are mandatory but I discovered a problem for the moment without solution and fatal for my project.
Customer Area is a private area where users can enter only after a logon process in order to perform actions on their files and at the end they exit with a log out process.
After the log out no way to enter again without a new login (this should be ok) but in the reality :
– AFTER THE LOGOUT YOU CAN ENTER AGAIN THE PRIVATE AREA ONLY BY ACTIVATING THE BACK PAGE BUTTON ON THE BROWSER.
I sent same message to Customer Area : any idea how to solve this problem ?
Many thanks for your support and best regards
Bob