Is it possible to stop thoughts in their track? Yes, we spiritual travelers specialize in that kind of thing. Meditation and mindful living gives us a little more control over our thought pathways, but this is only marginal I would argue. When the pressures of life really kick in, we are as much victims to our thoughts as everyone else is.
No, the way to clean up in your head is by changing your life. I spent years in sexual chat rooms and had all kind of sexual imagery floating through my head at that time. But when my spiritual path opened up, I simply figured that that stuff is not so helpful along a spiritual path. When a sexual image popped up in my head I simply asked myself, "does this mental image express love?", and almost always found out that it didn't. I did that for a while and the images simply stopped. Of course, the fact that I stopped participating in chat rooms, and avoided pornography in every form was essential in this transformation.
The same with fearful thoughts regarding competition at work. Ten years ago I felt that some of my colleagues were out to steal my spot in the lime light; I felt competitive vis-a-vis them and had many negative thoughts running through my head. But then I discovered a weird equilibrium of sorts; I realized that as my thoughts towards these perceived antagonists changed, so did their actions. Over time, I catapulted myself into a world of friendship and cooperation. Don't get me wrong, I am not saying that you should be naive and embrace the entire world as being your friend, but I would encourage you to deemphasize your conspiracy thoughts and just look for yourself what is going on moment by moment. Some former perceived antagonists become my friends, others simply disappeared from my life. You can clean up your professional life that way as well, one relationship and one conversation at a time.
"You are a loser"
"You will lose your job"
"People are out to get you"
"You are poor"
"You are fat"
"You will never find someone who loves you"
"You are just not cool as the others"
"How dumb you are"
Does any of this stuff sound familiar to you? Well, you can't simply say no to these negative mental loops. They are the subconscious expressions of your belief system. What you have to do is to change your belief system. You live in a world of abundance. You are loved, you are beautiful and healthy. The people you interact with are your friends. A spiritual path does this for you. Wall after wall of limitation, fear and scarcity gets torn down by life, and as it does, the chatter in your head calms down. Relax, there is little to do; just follow Her lead step by step. Soon you will feel easy like a Sunday morning every day!