Delete a cookie every time a user logins to portal page

I currently have a portal for webmail. When the user logs out of webmail or doesn't close the browser and another user logs into the portal, they are presented with the mailbox of the last user.

 

I am looking for an irule that deletes all cookies at the portal page to prevent another user from getting the last persons information. I am hoping this is the issue rather a persistence connection issue.