Revoke access from table - oracle database 19c

Hi Team,

We want to revoke select access for READONLY user on table xxttd_flex_value_sets_tab. But we are unable to revoke. Not sure who and how they gave access on that table to READONLY user.

SQL> sho user
USER is "READONLY"

SQL>SELECT
'Basic ' || vl.description
FROM
xxbps.xxttd_flex_value_sets_tab vs,
xxbps.xxttd_flex_values_tab vl
WHERE
vs.valueset_id = vl.valueset_id
AND vs.valueset_name = 'XXBPS_RESTAPI_VS'
AND vs.status = 'Active'
AND vl.status = 'Active'
AND upper(vl.value) = 'BPS AUTH KEY';

'BASIC'||VL.DESCRIPTION

Basic RkFQUkRJTlQ6b00zIzEyMzQ1