Thanks for the reply. They were running full time bot before I started to compete to them. I have no idea whether they'll start do that again if I stopped, it's fair to assume they will. If they want to show me that they won't do that again (sounds like I'm police), they can reset their reward points by self-trading on the opposite direction, then I'll remove them from my "bad guys" list manually, and start to compete "normally" . Anyway I don't think it's bad to set a barrier.