Mr. Deltoid is Alex's "post-corrective adviser," and he's terrible at his job. If he were any good at all, then Alex wouldn't act the way he does. His method of discipline—threatening Alex with jail—is totally ineffective. Since Alex is our only reference point in the film we can only speculate if this method of discipline is being deemed worthless in general, or only for Alex.
Mr. Deltoid also has a skeevy molester vibe, leering at Alex in his underpants and grabbing his junk. He's only in the movie twice, and although he pretends to be concerned that Alex might go to jail, he is blatantly happy when Alex does go to jail, as if he knew all along Alex's fate would be inevitable.