Wireless Access

We have some handheld devices that need to connect to our Aruba AP.

We noticed that they often disconnect and then connect to the other SSID from Guest WIFI. This guest WIFI is not from Aruba AP but another brand.

How can i force the handheld to stick to connecting to one default SSID ?

The handheld are android devices.

You'll need to configure the devices to only use the one WLAN.  You'll likely need to use some sort of management application.