Hi Ian,
You could use following code to sign out a user:
System.Web.Security.FormsAuthentication.SignOut();
CMS.CMSHelper.CMSContext.ClearShoppingCart();
CMS.CMSHelper.CMSContext.CurrentUser = null;
Anyway, there is a web part in Kentico
CMS intended for signing out users. So, you might find some inspiration how to log out the user in a following file
~\CMSWebParts\Membership\Logon\SignOutButton.ascx.csBest regards,
Michal Legen