Add 'CASCADE' after the user's name in the DROP USER statement : Drop User « User Privilege « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. User Privilege
3. Drop User

If the user's schema contains objects such as tables and so on.

SQL>
SQL> CREATE USER user1 IDENTIFIED BY pass1;

User created.

SQL>
SQL> GRANT CREATE SESSION TO user1;

Grant succeeded.

SQL>
SQL> DROP USER user1 cascade;
SQL>