Whenever we slap hands and fist bump before we roll, this signals to me that I should now begin attempting to submit you with all of the skill and willpower at my disposal. To give you less than 100% is to disrespect you, as if you are undeserving of my full effort. Or, worse, I am making preemptive excuses for myself, legitimizing my future loss to you because it is something that I have accepted as inevitable. If I hold back, the cowardly recesses of my mind will always be able to whisper to me, "pssst, that guy didn't *really* submit you. You could've bested him with that last 20%." The insidious voice of my ego will then pervade my actions, skewing my view of myself and my abilities. Far better to suffer only honesty, and deal with the indignity of loss with the only possible conclusion: that I must improve myself to avoid it. This endless cycle, the confrontation with and honest assessment of my own failure, is what drives me to become the most my potential allows. Every training match IS the world championship; the championship is not won on the day, but in the months, years, and decades beforehand.
With that said, I believe that my mindset was not actually what you are complaining about; I don't think you are annoyed when black belts tool you, for example. You are complaining about the far-too-common spastic white belt who is dangerously flailing his limbs with all his might as if you are threatening the lives of his loved ones. Little skill enters into his combat style. You are likely to get poked in the eye or smashed in the gonads or worse even as you submit him a dozen times in a single match. Now, I am fairly large so I am often able to subdue such an opponent forcibly and in the process gain valuable insight into dealing with the no-technique style of a neophyte. However, I have been faced with larger opponents (300lbs +) who attack with this mindset, so I understand completely. In my opinion, the thing to do is to discreetly speak with the instructor and let him know that Big Jimmy needs to be calmed down. As an instructor, I have spoken with countless novices whom I felt were shortchanging their own learning process by spazzing during every roll. I tell them to calm down, to think about each and every action before doing it, and to only do something if it is a technique they've learned in class. I remind them that tapping out is good, a part of learning, and not to be feared. I tell them that I myself approach every match with 3 to 5 specific techniques that I am attempting to use, and that I will place myself in positions to use them almost exclusively.
You can also send them to roll with an Enforcer (e.g., Adam Sachnov) who can thrash them and then calmly explain all of the above.