SQL Reporting :: Restrict Access To SSRS?

I need a reliable method to switch off users' access to SSRS dynamically. If you care about the reason, users are not allowed to access SSRS from home, but they are allowed access from within the factory walls.

I can generate a token or event when they arrive at work or leave, no problem, such is the sophistication of our security system.

So I can create a little .net app that pokes SSRS in some way and tells SSRS to allow that username to access reports. When the users leaves the premises, the .net app will prod SSRS to deny that username access.

I considered dynamically adding and removing usernames from the authentication section of web.config in the SSRS root dir, as in <deny=usernamelist />. But given the frequency of changes (dozens per hour at peak times), that seems too intrusive, as it probably causes the restart of the app.

I tried adding usernames to the ACL on the SSRS physical directory (Microsoft SQL ServerMSSQL.2Reporting ServicesReportServer) as deny reader, and for a few brief minutes I thought I had arrived at a solution, but for some reason SSRS decided to serve pages to denied users seemingly at random. Must be cached somewhere, although I can't for the life of me figure out why that would be happening seemingly at random.

I rather like the ACL idea from the perspective of ease of control, and if there's a simple thing i have overlooked in the way SSRS interacts with IIS and NTFS permissions, I hope someone can point it out so I can understand why the ACL seems to be mostly ignored.