when you link to the members area, have it create a brand new document and give it a secret handle. Then before the members page loads, have it check the name and if its wrong, just redirect them.
Try going to, http://24.16.245.89/about.htm/ . Because the handle is wrong, it goes straight to the homepage (and takes the file into account when loading the frame).
If someone really wanted to, they could get around it, but not an average viewer.
EDIT: crap, my IIS is down AGAIN! That redirector wont work because I cant run asp apparently.