There is no public API for this in Android. You can call a hidden method with this code:
B4X:
Sub UnpairDevice(Address As String)
Dim adapter As JavaObject
adapter = adapter.InitializeStatic("android.bluetooth.BluetoothAdapter").RunMethod("getDefaultAdapter", Null)
Dim device As JavaObject = adapter.RunMethod("getRemoteDevice", Array(Address))
Log(device.RunMethod("removeBond", Null))
End Sub