Cookies are limited to 4 KiB of information so I guess the best we can do is let users have a cookie that points to a list of public URLs with the things they don't want to see. (kind alike linking to a paste bin)
Now that I'm thinking about it, this feature sounds so useful it might be a good idea to standardise it somehow. Perhaps even with an IETF RFC.
As for the file format itself, I think that some simple JSON thing is enough. (let me continue in another toot)