Re: The Role of Sempai
Sounds like you and the other seniors covered what you can when you described "keeping them in check".
At the risk of being flamed, the other students are adults, and should, to a degree, learn how to fend for themselves. If the "dynamic duo" see enough people shying away from them, they might get the hint. Of course if there is a real risk of injury intervene, but shy of that...
If you lead class in Sensei's absence, you could take it upon yourself to REMIND everyone that you are all there in a spirit of cooperation (oddly my Sensei did this the other night, though the person who could have benefited most from that reminder was not there).
I know all too well the feeling of "Boy, I better talk to these guys before someone gets hurt, and put them on the right track"; I have needed to curb it often. But I learned that the lessons on the mat (good and bad) should mostly be left, well, to be learned by those working through them. Sorry if that seems vague.
As for Chris' comment above, I agree with talking to them, but would caution WHEN and HOW; if done on the mat, during class, in front of fellow students ESPECIALLY a junior to one of the two mentioned, it could be misinterpreted as a dressing-down or scolding. Tack (sp?) is required, I think.
And what Burt says above could also be the case.
Feel free to send me a private message and we can discuss it further.