A couple of things.....
there is no way to limit what channel a call comes in on. Not even a telco can pull a single channel out of a PRI for incoming calls. That's just the way it is.
As far as outgoing calls goes, this is quite funny. Avaya (UK at least) refuse to admit that a single channel can be removed from an outgoing group. They say it cannot technically be done. I laughed when I was told this as I've been doing this for years on numerous other systems. In the Line programming you will notice that you see all 30 (24 in US/Japan) channels listed and they're all in the same OG group. When you change one, it changes them all. What you want to do is make 29 in group 0 and the 30th in group 1 then allow only one user to make a call on group 1. In an early unreleased version of 3.0 Manager allowed this to be done but they "fixed" that. I've made complaints about this directly to them and they say it can't be done. But it technically can be done, they've just got to work out a way to do it. Once they catch up with the rest of the world, you should be able to do this. You still won't be able to stop a call coming in on that channel though.
Hope this clarifies things.