Yes, it can be turned off. I've done so, and can check how once I get home and get in-game. (Well, I can check for PS3 / PS4. I can only assume it'd be the same for controller with a PC.)

As to why turn it off, it's because you'll generally wind up "clicking" it whenever you try to run. Locking on may be workable if you could choose to do it on purpose, but L3 is a terrible place for it, because that leads to locking on right as you're in the midst of trying to run away from whatever major AoE is coming in. Before finally finding how to shut it off, I died way too many times as that lock-on changed my escape attempts into walking straight into the mob I was trying to get away from.