Thought stopping is a psychological technique used to deal with persistent, obsessive, and negative thoughts. It consists of having a predetermined, simple cue to break the unproductive thought process and return to more healthy thoughts. The most common cues are short commands (e.g., "stop"), or a physical cue such as snapping a rubber band on the wrist.
This technique has been attacked and defended by psychologists for decades. There is no consensus on when it should be used, who should use it, or what the success rate is. There are people building minor careers arguing that thought stopping simply does not work (and selling books explaining alternative methods), their articles accompanied by user comments claiming both that thought stopping was a complete waste of time and that thought stopping worked and the author is misguided.
There are few good studies on this, and they are somewhat wishy-washy in their conclusions. From what I can determine, thought stopping is low-risk and high payoff, but doesn't work for many people. When it does work, it does not work by banishing all negative thoughts, but by breaking the flood of negative thoughts so that they exist at a manageable level (this is in direct contrast to what many pop-psychs claim, i.e., that thought stopping is an analog to the white bear experiment -- if you think of the white bear even once, you lose!)
YMMV, but some anecdotal data indicate that pairing thought stopping with self talk and breathing exercises works for many people. It may be beneficial to try multiple different cues to find what works for you. It is also wise to view thought stopping as a management strategy, and not a cure. If it makes matters worse it is not a successful strategy, and should be discontinued.
Brevity Quest 2016