According to me,I think so lord Krishna could stop the Mahabharat. In the field of Kurukshetra, Arjun at first refused to fight battle as he thought that it would be a delinquency to fight with those who are his own brothers,the kauravas. Then Krishna told him that at the field of battle, there is nothing precious then victory, and there is nobody,no family member,no friend,no companion. Only the member of the opposition army.Nothing else then this. Krishna told him that Victory is not in his hands,but in someone else's hand. There is onething which is only in control ,that is karma or the work,struggle he does(in hindi,if you didn't understood,कर्म हमारे हाथों में हैं , परंतु उसका परिणाम हमारे हाथ में नहीं है,बल्कि किसी अन्य के हाथों में हैं,इसलिए परिणाम की चिंता किए ब गए कर्म करिए I) And Arjun follow the path of his friend and teacher and did what Krishna said him to do. If Krishna had not told Arjun about all this,he had not fight the great battle of Mahabharat.
In other hand, According to @Kartik Anudeep, BhishmPitamah could also stop the Mahabharat if he had not battled from the side of kauravas,For him it was from Hastinapur.

Shri Krishn Ji and MahaPitaBhishma Ji were the only persons who were not able to stop the war..but ot didnt happened ...ShriKrishnJi didnt stopped as he wanted arthat Dhramma should get victory on Adhramma ....and Bhishma Ji didnt stopped as he was Vachan badh that he wouldn't create any problems in the decision of the King at that time ..and be the servant of the King.