Because you are using a window which was not created with javascript (i.e the one holding beforeclient.html), you will always get asked for confirmation before closing, so the easiest way to avoid this would be if you could make beforeclient.html be opened in a new window.
In Javascript 1.2 you can close windows unconditionally, only if you have the UniversalBrowserwrite privilege,
which I have not investigated but sounds convoluted!
I don't know if this is possible in the site you are creating, and I realise you probably don't want to do this.
The thing is, by the sounds of it, if you are closing the original window, then you are cutting yourself off from the rest of your site though, so to get back in you will need to open additional windows.
But it comes down to the fact that you can only close windows created with javascript,without confirmation.
Personally (without the benefit of all the factors) I would just open beforeclient.html in a new window, as a stepping stone, because you are moving into a completely new window anyway when you go to client.html. This way the close methid works sweetly, no probs.
Let me know if this is un-feasible!
Ben