Maybe the way to handle this is to audit the number of cookies periodically. And, say, if you're over your preferred limit, clear all BUT the required cookies. Or maybe just clear the oldest of the cookies that you don't recognize.

On my own site, when I've seen a large numbers of cookies gradually appear, I've done a search on a few of them. It turns out that my Ad Network as leaving little (or not so little) cookie leftovers.