I can't answer for support's speed in working on this, but I can say that it's very difficult for a host to completely clear out spammers. It can be as simple as someone forwarding incoming mail (to an LP account) over to gmail, Yahoo, etc. The incoming spam goes along for a ride, and gets the LP server blacklisted as the source. As you note, a compromised site (often running a CMS) can pump out spam, and it doesn't take much to get blacklisted, so the owner may not even notice a slight increase in email levels (so long as they stay below 300-400 per hour). Finally, accounts for the purpose of out-and-out spamming can happen, but LP can notice those fairly easily.