Yes. It is possible to turn off the build in AP on the 651 controller. You can find the instructions in 6.1.3.4 Release Notes and I have pasted the same below:
In the CLI
1. Create a dot11g radio profile and disable the radio
(Aruba651) #configure terminal
(Aruba651) (config) # rf dot11g-radio-profile disable-radio
(Aruba651) (802.11g radio profile "disable-radio") #no radio-enable
(Aruba651) (802.11g radio profile "disable-radio") #exit
2. Apply the radio profile to a specific AP.
(Aruba651) (config) #ap-name <ap-name>
(Aruba651) (AP name "<ap-name>") #dot11g-radio-profile disable-radio
(Aruba651) (AP name "<ap-name>") #end
3. Save the configuration
(Aruba651) #write memory