Function revoke 

pub fn revoke(
    owner: RoleIdentity,
    acl: &mut Option<Vec<ObjectAclEntry>>,
    grantee: Option<RoleIdentity>,
    grantor: RoleIdentity,
    grant_option_only: bool,
    cascade: bool,
) -> Result<(), SQLError>

Remove the privilege or only its grant option; revoking what the grantor did not grant changes nothing, and ExecGrant_* warns only when the grantor holds no grant option. Privileges granted through a lost grant option require CASCADE.