Suppose I am an admin of a database instance. Now
I need to specify that user A has the right to create tables
in his own schema, but not anywhere else.

In order to let user A create tables, i grant createtab to that user.
However, that would enable the user to create tables in other schemas
as well (except those sys schema).

Any way to restrict the users from creating objects in other schema
than their own?

thanks