How do you stop an AJAX web service from keeping a session alive?

Using ASP.NET 2.0, AJAX.NET 1.0 and C# on a Windows 2003 Server platform (both for development and for production), how do you stop an AJAX.NET page polling a web service within the site (not on a separate site) from keeping the session alive. The site uses an internal web service rather than an external one, as the site will ultimatly run https.

The page should time out if the user does nothing for the time out period even if the page has a call to the AJAX web service.
 There are numerous pages on keeping a page alive, but seemingly none on stopping a AJAX poll keeping the page alive.
 
...