Yes, it is possible to disconnect user from geodatabase from Oracle side HOWEVER it may leave an orphan entry in process_information table in SDE schema (or any other user schema if any). You can read more about this table on following link:
ArcGIS Help (10.2, 10.2.1, and 10.2.2)
Entries in this table without actual connection to the database may make 'ArcGIS' think that there are still connections to the database. So we cannot say that a user was successfully disconnected from the 'geodatabase' unless a row corresponding to it is removed from 'process_information' table.
Having said that the other way would be to disconnect the user from Oracle side and then manually delete its corresponding row from 'process_information' table. But ESRI may not recommend this approach.
On the other hand, end user's concern about 'ALTER_SYSTEM' privilege is also correct since it enables user to alter other things (apart from disconnecting user) as well.
Hope this helps!