I had this issue today and I found this useful script:

output to "revokepub.sql"
without headings
select "revoke all on ", -- Revoke all privileges
trim(tabname) , -- Table name from systables
"from public;"
from systables , systabauth
where systables.tabid = systabauth.tabid
and grantee = "public" -- Select table names where public has privileges
and grantor = USER -- Only revoke the ones you have privilege to revoke
and systables.tabid > 99 -- Don't revoke the system table privileges

This script creates another script (revokepub.sql) that revokes privileges from all tables in a database.

I will run revokepub.sql on a busy database and my question is if it will cause any performance issues or if it will fail because some tables will be locked.