It is hard for us to judge this guy since we have not seen his Kendo yet. What rank are they ? What is your real complaining ? Do they have bad form ? Do they hit your unprotected spot ? They can be bad because they are still beginner. Some of them can be very aggressive and use lot of power because they don't know how to control their strength. I think it is upto Sensei to make any decision for this. You should go talk with other sampai and sensei about this.