Yeah, your short term answer is to use a site like proxify to get around the problem. If that doesn't work, clone the mac address and power cycle that sucker. You'll want to type in your router IP to the address bar of a browser to do that.
If you've the gift of the gab, call comcast and explain that your neighbours room-mate was recently evicted for stealing the neighbourhood wifi bandwidth, which is why you're banned from all those sites and are now in need of a new IP address. Apathetic grunts in IT support will more readily assist you if you can brighten their day, and they can easily identify with the numpty neighbour story.
I don't suppose IPconfig/renew would be of use here?