Wireless VoIP SIP phones keep changing SIDs

[ChrisOD]

    I have two HP 830 wireless controllers and some MSM430 APs.  I have several SIDs and am trying to get a stable wireless VoIP environment using ShoreTel VoIP system.  The problem is that as the VoIP phones roam, they jump to a SID that is not configured for them and then have no connection to the ShoreTell controller.  The unwanted SID is broadcast and the desired wireless service is not broadcast.  The desired wireless service is an 802.1x with WIndows RADIUS server network.   Is there a way to blacklist or block a MAC from joining a network so that the phone is always attaching to the correct network or somehow associating a MAC to a particular Wireless Service?  If you could point me to a reference link that would be great.  I just want to know if this is possible or how other people handle this situation.