Becky's right about verbally reprimanding too... I forgot to mention that... something like 'quitit!' or 'Aaaaat!' (my favorite nondescript reprimand)
And I'd recommend the stud chain - ~not~so that you are really hard with it, but rather so that you can be very subtle. Pushy horses can be pretty strong on just a halter, they like to yard you around, and I would teach him to respond to 'less' muscle by using the stud chain to give him a correction.
I couldn't figure out how to spell that, but now that I see it, "that's it". That's what I say in those circumstanses:) That's what I meant by a "correction noise:)"
Becky