Give users a choice either ads or limit a bandwidth. I use Evernote rarely, just to memorize some stuff I need to get back later. I currently have it on 5 devices. 2 laptops with dual OS Win/Mac and a desktop PC. It's gonna be fairly inconvenient to manage these. I think I'll have to switch to web-version or seek for alternative.
If I understand it correctly free accounts are generating too much traffic. But you don't limit web-version to two devices? What if everybody starts using web version, won't this be even more traffic?