To delete user schema geodatabase, you can use the following command:
sdesetup -o delete -d ORACLE11G -u oracle_child_geodatabase_user -p password -i 5151:oracle_geodata_user
important remark: you need to specify the port and user schema geodatabase schema.
user schema geodatabase has been configured in Enterprise Geodatabase as a child Geodatabase. The Master Geodatabase is “sde” schema. ESRI is providing this architecture for organizations that require limited number of customers needs, also for data isolation purposes (each child geodatabase will have its own super account similar to “sde” where system tables are stored in it). Not only that, this architecture will also save cost since you will only need one License File for your complete geodatabase.