Deleting Database Users 
When you delete a database user, all schemas, synonyms, and tables of that user are also deleted, along with all related indexes, privileges, view tables, and so on.
Note though that deleting a user of the DBA user class does not affect any database users created by this user. The database system administrator becomes the new owner of these database users.
Prerequisites
The database is in the ONLINE operational state.
You are logged on to the database as database system administrator or as a database user of the DBA user class and are the owner of the database user you want to delete.
Procedure
This function has not been implemented in Database Studio yet. Use the corresponding SQL statement instead.
More information:
SQL Reference Manual, SQL Statements Overview
More Information
Database Administration, Database Users