Bots are simply programs that act like people. Thus, there is no way to really stop them from existing. They simply do the same steps as people and they don't have anything that truly identifies them.
As to bots that do help. @cheetah bot by @anyx is responsible for keeping a lot of spam and plagiarism off of steemit. Before it existed there was a period where spam posts were 90% of what was flooding the new posts.
They can be abused as well, but there are positive uses for them.