I believe you can identify a proxy by polling it back, and if it's anonymous or high-anonymous, deny. Transparant proxies (like those in most businesses) can still come in.
Also, there are many publishers of proxy lists. Scrape those lists and put together a daily floating Denied IP List.
I don't know if IP tracking blocking is the easy way to do this. If a person has a legit reason to be on FR he would be posting from probably no more than three traceable, static IP addresses. These could be individually confirmed as not being proxies at the time the person signs up or requests another IP to post from. This would give the admins time to check approve or deny the IP/poster. If it wouldn't be too much the admins could talk with someone signing up via their sign up e-mail, I would guess that alone would stop some disrupters.