Disconnect SSL connection after user has logout

Dear All,
I need a little help here.
Currently I have a website which is not under secure (E.g non-SSL pages).
If the user clicks login, the system will verify whether the login page is a secure page. If it is not, it will had a change of the URL to "https://" instead of "http://" which brings the website to be under secure.
However, if the user logout, my website will disconnect the SSL connection.
Now my problem is, how to disconnect the SSL connection in C#?
I tried searching googles, forums, etc for about a week and most of the replies are by using "FormsAuthtication.SignOut()" but it does not work in my website.
I include "FormsAuthtication.SignOut()" and redirect to "http://" page, it give me an error:
"The page must be viewed over a secure channel
The page you are trying to view requires the use of "https" in the address.
HTTP 403.4 - Forbidden: SSL required
Internet Information Services"
Please give me advie on this.
Many thanks.