How to stop outlook connecting to a particular exchange 2010 CAS?

We have 2 Exchange 2010 client access servers in a CAS array, this works fine.   Outlook clients and systems point to the CAS array address.  Both are also Hub Transport servers

We have 1 other CAS on the same network that is solely used for OWA.  This only has the Client Access role.

Occasionally outlook clients will try to connect to this 3rd CAS rather than the proper CAS array.

How can I stop clients connecting to this 3rd CAS? I dont want clients to ever use this instead of the array.

I was thinking of using the windows firewall on the server itself to reject the outlook clients having access to it.  These outlook clients are all within a contiguous range of IP addresses so I would just block that range.  

But is there a better way to stop it advertising itself and taking requests away from the CAS array?
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Miguel Angel Perez MuñozCommented:
Sometimes Exchange change CAS function wantonly.
You can check where CAS function are before blocking anything on firewall:

Get-mailboxdatabase “DatabaseName” | fl *rpc

This show you per database what CAS server are using your clients, to change this you can use command:

Set-mailboxdatabase –identity “DatabaseName” -RPCClientAccessServer “ServerWhereCASMustBe”
AmitIT ArchitectCommented:
In this case, you need to setup another site in AD and move or install server with CAS role. Then only you can remove it from the mix. Else in same site, it is a default feature.

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Malli BoppeCommented:
Remove the CAS server role or  create a new site as said above.

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