Okay, you might have mentioned this earlier to clarify the situation. It's beginning to make a bit more sense now. If your sensei isn't around (especially if he/she is absent for a long period), then the most senior sempai is the one in charge and the one who needs to take responsibility for issues like this. You should definitely bring up the issue with him and have him observe the troublesome newcomers. If they somehow seem to be able to tone down their rough play whenever the most senior sempai is watching, then you must tell him about this also. It may require that the senior sempai look out of the corner of his eye to catch these guys in the act, so to speak. My other question to you is, does any other sempai have an issue with these guys? Surely, if they are as rough as you say they are, some other sempai must have done keiko with them or at least seen them. How many yudansha do you typically have at any given practice?