I m trying to resolve a problem of logging out successfully by allowing the user to click a link. 

I have multiple portals in my site and i had to remove the offical Login-Register control because if a user was on a child portal and wanted to register or login they would click on the register link and it would register them to the child portal and same with login if they clicked on the login link on the child portal it would log the user into the child portal that was no use to me.

So my removing the Login-register control from the page and just creating a Login hyperlink pointing to the main portal login page and same for the register this works great , now when your on any portal when you click login or register it will bring you the main portals pages.  The issue arises when a user who is successfully logged in wants to logoff. 

I have the logout hyperlink pointing to the URL in this format :> http://r8yourpolitician.co.uk/tabid/36/portalid/0/Logoff.aspx this works I get logged out and i get redirected to the right page.  However if i click on "login" again it automatcily logs me in without asking for username or password.  It behaves as if i was never logged out. 

Of course if you clear your cookies and cache you will not get this but I expect when a user logs of they should be logged of because if another user came after them and used the computer and cliked login so they could use the site it would log in the last person who logged of . 

Any advice on how I can solve this security risk , how can make sure that user is logged of and and cannot log in unlesss they provide credentials.

kind regards