The host is saying that the IP used to send the spam is now blacklisted. That means in a shared environment everyone who uses the same IP is also blacklisted.
This can be a big issue. More and more email providers are relying on Real Time Blacklists to determine if an email is delivered or not.
So, someone who has never sent a spam email can suddenly find that their business emails are not being delivered to their customers who use the likes of hotmail, yahoo and others. Simply because of the IP they are using.