What is the standard way of only allowing a single login during an Oracle 9i database export to prevent others from modifying the database during or after the export?
You can use restricted mode to keep everyone off who does not have RESTRICTED_SESSION privileges:
ALTER SYSTEM ENABLE RESTRICTED SESSION;
This does not prevent other users with this privilege from logging on.
Why would one need to? An export in CONSISTENT mode would take an export as of the SCN when the export kicked off; functionally equivalent to:
SET TRANSACTION READ ONLY; SELECT FROM TABLE1; SELECT FROM TABLE2; . . .