Depending on the specifics of your application, you might want to think about different angles.

For example: If more than one distinct page needs to only take one user at a time, I would think about creating a new table with a record for each of these pages. This way, you can set a page as logged-in/in-use using the user's unique ID when someone logs in or access the page. When the user logs out/leaves the page (or if their ASP Session expires- users do not always log out cleanly!) you can "unlock" the page again. Not only that, you might reduce database load by searching specifically for the page record rather than any user with a logged-in flag.