SharePoint sign out not working in Google Chrome

We have an Extranet SharePoint environment published in Internet for our clients. The issue is that if the user is using the Chrome browser and even if they select “Sign Out” or just close the browser, their session remains still open and the next person to visit that site is logged in with the previous user’s credentials. Many of our users will be logging onto this SharePoint site from public computers so this is a huge security risk. Is there any solution for this?