One way would be to use an Application variable as a flag, so when someone goes to the data entry screen it checks the status of the variable, eg Application("PageLocked").
If the variable is False, eg, then no one is using it so you set the variable to True to "lock" the page.
When someone else goes to the page, if the variable is True, you redirect them somewhere else ("page-in-use.asp") so they know that someone else is using it.
When the "active user" has finished and leaves the data entry page, you set the variable back to False, so someone else can get in.
You should also put something in the globas.asa Sesssion End procedure that sets the variable to false when the user times out, in case they didn't log out properly.
You should also provide an Admin "back door" to reset the variable and kick people off, Just In Case.