So you are saying that when they are connected to the LAN and have their wireless connected - that the user traffic is egressing the WLAN and not the LAN? This would be entirely the clients (machine) control - and should be selecting LAN preferentially. (but you knew that)
Since there's literally nothing the AP can do to influence this, I'd be looking more closely at the PC's - are they getting proper IP address, VLAN and routing on the LAN side? The acid test is having one connected to both and then disabling/re-enabling the wifi to see how the connection changes. Then basic "PC cruft" troubleshooting like deleting WLAN/LAN profiles, etc.
More details would help as well - OS of the clients, version of ZoneFlex running, etc.
And you can try this (if Windows..) - http://www.howtogeek.com/117083/how-t...