How to cut off the on-board antenna in BLE shield 2.1?



I’m new to arduino and BLE shield. I am working on a project that I want to receive the RSSI value from the BLE shield,but I don’t want the on-board antenna(nRF8001, right?) to work.I’m using arduino mega 2560 with it.
I have soldered an directional antenna on BLE shield with SMA connector, and I need to make sure that the signal sends by the directional antenna is not interfered by the on-board antenna. I have read a lot of topics, but none of them mentioned about how to cut off the on-board antenna.
Is that possible to make only external antenna to work?(like modifying code or something)
Or is there any physical way to block most of the signal sends by the on-board antenna?

Thanks a lot for help!