Did you go to him in a group? If not, this is what you should do. Individual complaints can be ignored, but when a group of people come to you together, it has a different impact (that's why interventions are successful). If you have consensus among the sempai, you shouldn't have any problems getting them to do this.
Good point. We will keep an eye on this guy and the next time something happens the group approach sound like a good idea as stated by Abrams Sensei and yourself.