How to disconnect voltage from BeagleBone Black expansion pins?
For multiple signals a bus switch like the TI SN54CBTD3384 http://www.ti.com/product/sn54cbtd3384 would be a good option. It has DIP package which will be easily solderable. Power it off the 5V supply to the beaglebone (which means that if the beaglebone isn't powered then signals don't get through and then connect the reset to the enable pin. 
This part will also level translate to prevent any voltage greater than 3.3V getting to the beaglebone.