There isn't a way outside of creating a separate SSID. Clients make the decision when to roam and what APs to connect to so it may be something odd specifically with that client. Do you have minimum transmit rate on (in Configuration->Wireless->AP templates and then select your AP template)? Having that on could possibly influence that decision (I would turn it off to test).
ADTRAN Product Support Engineer
Thanks for responding Evan. I saw that the SSID DynamicSteering option was turned on so after unchecking it and applying the config to the APs, everything has been stable for the past day or so.