Tape the vents for a partial solution, but I find it is a crappy compromise.
I now have a Bern that I really like. The vents are smaller - it is more of a "multisport" type helmet with a sort of "street style" that makes it somewhat cool looking.
I've gone kinda nuts with reflective tape on that helmet, with all of its open space and all. I probably look like a space station module on final approach in someone's headlights.

I wear an OR balaclava which gives me primary coverage over the top of my head to hold most of the heat in, and I wear a polar headband if I need more on my ears. I've had some rides down to about 12F with this setup and I've been toasty, perhaps a touch TOO warm.