Amumu's passive reduces it by 40, eve's ravage reduces it by 26, Ryzae can reduce it by 90 with good timing and some luck, exhaust reduces it by 10, and magic resistance reduction stacks with your teamates.

So if you put Amumu, Ryze, Eve, Exaust, Abyssal Scepter, Fiddle together you can get
40+90+26+12+20+16=204. You can definitely get someone below 0, its just not all that easy.

Edit: Note that multiple exhausts and abyssal scepters will not stack with themselves (aka 2 exhaust is still only 10 magic resistance reduction).

Don't forget Soraka's Starcall (-8 MR/cast, stacks 20 times), or Karthus' Wall of Pain (-35 MR at lvl 5).