You can just break it into pieces and throw it in the wastebasket. Once it’s polymerized, acrylamide is no longer much of a toxin.

The simplest thing is to break it up with a spatula or plastic pipet until the pieces are small enough to fall through the mouth. Then you can just rinse out the little gritty things left behind. If the flask is so large a normal spatula or pipet won’t reach, then you’ve got a problem. You might try some acid chromate cleaning solution. Qiagen sells a kit for recovery of nucleic acids from gel fragments that actually dissolves the acrylamide. It’s designed for fairly small volumes of acrylamide, though. I always thought the active ingredient was a chaotropic salt like LiSO4 or LiCl, but I don’t know for sure. I guess you could try heating in 1N NaOH or 1N HCl to see if you can hydrolyze the amide links. Maybe KMnO4 cleaning solutions might work?

Yeah I am not sure that using chaotropic ions in a large quantity is good idea, because you should not just get rid of them down the sink afterwards…
Another thing I would suggest is to just put something with sharp/pointy sides that could fit through the Erlenmeyer (yes the name is the same in English and french) mouth and bounce it inside until it breaks up the acrylamide.