Lulu's Polymorph changes the enemy into a squirrel for a few seconds, slowing them down, and also preventing them from attacking and using abilities.
It seems to cancel some abilities, like Death Lotus from Katarina, or Warwick's ultimate.
However, there are a lot of other abilities which are not cancelled, and keep going even during the polymorph. This leads to strange effects, like for example a squirrel running after you with a circle of fire around him (Mundo) or a sword spinning (Garen). It also doesn't cancel Fiddlestick's ultimate, once he teleported.
You would expect someone changed as a squirrel to be unable to do such things. So what exactly can and can't be cancelled by Lulu's Polymorph?