Simplest way to "stop" a motor

I didnt think this would be this best way either, I dont think he has actually done this (my program is Mechanical design, so most of the professors know very little about electronics/are hesitant to allow us to have a project that utilizes electronics)

As is, I am still planning on using the Hall Effect sensors, I was just hoping someone had some sort of method that I had not found yet.

jake