What rom? On aokp, using a root file explorer, go to /system/usr/keylayout/sec_touchkey.kl open it with a text editor, delete the text inside and then save the file. Make sure your nav bar is enabled and then reboot. Once rebooted the keys will no longer work but may light up, to turn off lights, go to settings > display > backlight > custom settings, check use custom settings, go to edit other levels, in the buttons column set all levels to 0 click save and they will turn off, you may need to reboot for the settings to stick. Once both of those are done the capacitives will be fully disabled, no lights and pressing them does nothing. Using es explorer to delete the text in sec_touchkey.kl will automatically create a .bak for the file so if you decide to reenable them you just delete the .kl and delete .bak from the backup and reboot and the touch function will work and reverse the settings under custom backlight to get the lights back.
Sent from my SPH-D710 using xda premium