Yes, it's the clients choice to which AP (BSSID) it will connect, however Ruckus has a SmartRoam feature you can set to help with roaming. Read about how to set it up here
https://support.ruckuswireless.com/an...
Keith suggested you use a higher minimum rate, not lower the transmit power. By doing that you hopefully make the client think of searching for a new AP faster so it doesn't hold on for deal life at speeds of 1Mbps, but rather at 5,5Mbps or even higher. It's available from FW 9.6 if I'm not mistaken.
Tell us FW version please? Are you tunnelling traffic to the ZD maybe? What kind of authentication are you using? What are you using on for wireless connection from CPE to the base station (WiFi, WiMax, 3G, Microlink,...)?