Select Privilege

You can revoke the Select privilege on a sequence from another user, from PUBLIC, or from a role. Select privilege enables a user or role to use the sequence.CURRVAL and sequence.NEXTVAL in SQL statements to access and to increment the value of a sequence.

The following REVOKE statement cancels any Select privilege that was granted individually to user mark on the cust_seq sequence object: 
REVOKE SELECT ON cust_seq FROM mark;