Here answer depends on whether you want to revoke system privileges or object privileges.
There is syntax change for the both.
But as you are logging through sysdba i am guessing that you want to revoke system privileges
.
revoke sysdba from user_name;
Here replace user_name with your actual user. like
revoke sysdba from nagendra;
Updated :
And to revoke all system privileges
we can use :
revoke all privileges from user_name
object privilege
means previleges on tables
procedures
functions
and to revoke this use :
revoke all on object_name from public
Example
revoke all on nagendra_table from public
This will remove all existing privilegs on table nagendra_table
from all users