I’m somewhat new to 11i…
Is it possible to drop an application user, or is it usually sufficient to set their end date in ‘Effective Dates’ on in Security->User->Define?

As far as I know it’s not possible, you have to set the end date as you’ve mentioned

You can only disable a user.
Oracle applications populates the user id in almost all the table as last_updated_by and created_by.
Deleteing a user will create data intergrity issues.