LOG OUT OF ACCOUNT AFTER A CERTAIN TIME

GOOD DAY.
I WISH TO SET UP A LOG OUT FUNCTION FOR THE ACCOUNT HOLDER TO BE LOGGED OUT AUTOMATICALLY AFTER A CERTAIN TIME.
THE REASON FOR THIS IS IF THE ACCOUNT HOLDER/ LISTING AUTHOR DO NOT LOG OUT WHEN HE EXITS THE WEBSITE, HE NEEDS TO LOG IN AGAIN TO ACCESS HIS ACCOUNT.
AT THE MOMENT YOU CAN EXIT THE SITE AND EVEN AFTER A WEEK, WHEN YOU VISIT THE SITE AGAIN, YOU ARE AUTOMATICALLY LOGGED IN. (LOGGED IN TO THE ACCOUNT OF THE LAST LOGGED IN USER)
IS THERE A WAY TO CHANGE THIS TO PREVENT UNAUTHORIZED USERS FROM ACCESSING THE ACCOUNT.
I TRIED IT ON FIREFOX AND WIDOWS EXPLORER AND ON VARIOUS COMPUTERS SO THE PROBLEM CAN NOT BE IN THE SETTINGS OF THE COMPUTERS OR THE OPERATING SYSTEMS.