A couple of possibilities for you:
Disable the upstart job that starts mythtv-backend. Upstart has service supervision, so if the backend keeps dying, upstart will restart it (kind of like monit or daemontools).
Rename the mythtv-backend binary. Then you can launch it from a terminal with debugging turned on. I'm not in front of my system right now, so I can't say for certain what the binary is called. I think it's mythtv-backend.
Hope that helps!