Depending on where we are, our way to get a horse's attention off of something that they do not need to be studying is to give them a reason to concentrate on the rider or the place they are. I usually take one off trail and go through the roughest brush, dead-falls, rocks or ??? That is close by. Same is true if we are riding together and one of us goes ahead out of sight. We just go up and down steep embankments or into the rough.
If the horse is broke enough or there is no really rough ground, we go to 'leg yielding' exercises. If the horse starts to even 'think' about getting out of control or nothing else works well enough, I just take its head away and let it stand there with its nose on my knee. A really GOOD 'one rein stop' that has been thoroughly taught ahead of time is like having a 'off button' installed in any horse. If it works (which it does) in OTTBs that have horses charging past them, it will work in any situation.