Is it possible for the issuer to control the maximum balance of (individual) trustors?
Haven't tried by myself, but it looks like you will be able to control the maximum amount with AUTHORIZATION REQUIRED flag. If you set this flag on the issuing account, each trustline requires your approval with ALLOW TRUST operation. You can approve only those trustlines that match the specific criteria (check the
limit parameter in your case).
It should work in theory, however, I'm not sure if a user can change the trust limit on the already authorized trustline.