well, you can fight them, join them, or try to understand why they are being the way they are. are they doing terribly in lane? did they get countered and try to build aggressively when they should have built defensively? are they just trying to do a fun build or champ in a role, or reacting in a hostile way to someone who is? there are times when you can't do anything to stop them from running it down mid, but if all you want is just to win, go play against some bots. try to act positively and calmly towards them, reminding them what they should be focusing on.
or you can go the route thats a bit more sinister and try to push them over the edge a bit more, such as stealing kills from them, stealing CS from them, pinging '?' when they die, reminding them that they can be reported for harassment, all while asking them to stay positive, just so you have all the more evidence to report them for if they break even further.
or, you could just tell him "KYS" and join him with the toxic behavior, running it down mid together. maybe your whole team might follow next. (/s)
if its a "toxic" mechanic or champion, try to find out how to play against it. i've seen champions from zed to ahri to rengar to tryndamere to garen to darius to anyone called "toxic". 90% of the time its "I don't know how to play against these guys because i don't understand the weaknesses in their kits and i don't want to learn or change my approach".