Re: The Role of Sempai
If they are endangering people, or not training in the school approved methods for that aikido class, then your sensei or you have to speak to them. I would even go so far as to have your sensei write them a letter explaining it. That way if it gets to the point where they have to be expelled from the class, no legal/discrimination issues come up.
If they don't immediately change their ways, boot them. Aikido isn't for them, they have other avenues to train if they choose. Flat out tell them that what they're doing isn't Aikido. If they want to do something other than Aikido, fine. They can do it elsewhere. You absolutely can not accept the risk of endangering people beyond the normal scope of the class. Especially in a college environment.