ISP blocked due to abuse?

Well, it seems today that I'm now unable to post anything over at the 4chan imageboards, otherwise I get this message:
"Posting from your ISP has been blocked due to abuse. [More Info]"
I can still browse the site, all the images and threads load fine, I just can't post. What gives? I've never posted spam or porn.

Portuguese here, my ISP has been banned for almost a year now. I do however manage to post on the imageboards once every two weeks, but the next day it's already banned again :(