I am working with a customer who has a Mitel SX100 and has requested that 411 be blocked. Can the Mitel do this and if so what software version do they need to be at?

If you have 216 or lower software you must use toll control to restrict certain numbers from being dialed(this is not too easy). If you have 217 or higher software you can use ARS to resrict numbers(this is way easier).
Hope this helps.