spamhaus is a big bad list of spammers. the problem as I see it is that once spam originates from any network they blacklist that entire network - immediately the spammer moves because of the blacklist, then it takes an age for the network owner to get delisted.
ask them if they can still pass spam messages on to you, but with a modified subject - e.g. [SPAM] Blah Blah - that way you can sort it yourself.
I'm with webattention.net and dont have any problems.