Is it acceptable to bully in the name of stopping bullying?

Ooh, a very interesting question. Off the top of my head, my initial reaction was:

"No! That'll just take you down to their level!". But then I thought about it; thought about how bullying is actually stopped by teachers or any other type of authority.

It all boils down to how you define bullying. The first definition that I will use will be: "Use superior strength or influence to intimidate (someone), typically to force him or her to do what one wants", which is the definition google provided to me. If this is how you see the definition of bullying, then yes, you would actually have to BULLY, to stop others from bullying, as you intimidate them and force them to do what you want them to do.

The other definition I'm going to discuss is Wikipedia's "Bullying is a form of aggressive behaviour manifested by the use of force or coercion to affect others, particularly when the behaviour is habitual and involves an imbalance of power" definition. (Link of the definition can be found in the related links). Contrasting from the previous definition of influencing with superior strength, this definition concentrates on the aggressive side. If you think about it, when a teacher confronts a bully, they talk with an understanding and controlled voice. Shouting at children or bullies will not work, since they usually come from abusive households, and being shouted on is almost a daily routine for them. The second part of the definition, where it states that it is "habitual", this also distinguishes from the first definition and is definitely not the appropriate way to stop bullying.

So, it depends on how you define the term bullying, and how you plan on stopping bullying, as there are many methods and definitions for both.