You could point out their flaws which I'm sure they have and make sure that they now how much those flaws about them stand out and such. That's what I have at the moment.

What actually worked in my case was saying to the kids 'Boy - if only you could see how other people see you when you do that...." and then I just shook my head and laughed softly. That made the little baskets so paranoid they worried more about themselves than the object of their torture.

Unfortunately, everyone (The bully) gets it back in some way all on their own through someone elses cruelty... You're just usually not there to see it. I'll tell you what I tell my own children. If they are doing it to someone else, they are probably having it done to them as children really do live what they learn. I always tell my kids to be strong and self sufficient and not to allow the things that other people say to them to effect them internally because in the minute that you allow someone else to take that from you, you begin to internalize it and then believe it and that is essentially the point of the bully. We are all tanking one another at all times, in any interaction simple or complex, in that we are all recepticles for either taking from, or providing to another in leiu of energy. Mental emotional and physical. Tell your children to cultivate and realize compassion for that child and help them to understand that nothing anyone ever says should hurt... If it is not true. It is not true unless they believe it themselves. Cultivate self confidence and self awareness in your kids and no one can convince them of anything even through bullying which actually is a social tactic meant and designed to break down someones defenses and force them to accept an outside influence as their inner truth. It is a game... Make them aware of it because it never ends. It just becomes far more subtle as we grow up and experience it in the workplace. The adult playground...

Find out who their parents are and call home. Tell their parents what is going on. You will find out one of two things. Either they have good and caring parents who will discipline the kids and make sure it doesn't happen again, or they have idiot parents who will pin your ears back for suggesting that their little darling could ever do anything wrong. If they have the former type of parent, the problem is solved. If they have the latter, I'd talk to the police about it. We had some kids like that in the neighborhood. When I talked to the police about it, they'd already had several run-ins with the kids. When I talked to the mom about it, she seemed sympathetic and said she'd take care of it. When they moved from the area a few months later (quite possibly because of all the scrutiny they'd been getting, the city had to send two dump trucks to clean up all the bicycle parts they'd left at the curb when they moved. It seemed the answer to several years of bicycle thefts in the area had been found. Mom had to have known about it, as you can't not notice two dump truck loads of bicycle parts lying around the house. So mom was complicit at some level. In that case, the scrutiny of people around the neighborhood quite possibly made them leave.

I threaten them, if they did it again, I will pay someone to kick their butt, they stopped, LOL.

They are bullying to feel important. If you cut down their importance and maturity, they will feel embarrassed and get a taste of their own medicine. I am not saying that you ought to bully and taunt these kids! LOL! I am just saying, that given that you are mid 20's.. they would probably be greatly discouraged if you were like, "Wow. I thought you guys were a lot older than that. Guess you haven't really grown up just yet. Oh well, and then shrug".... You might just disarm them.