You have all the right to mark it as SPAM of course. What I'm trying to get across here is that IP blocking is a bad solution. There are many cases where several domains share the same IP, specially with the shortage. So if domainA.com sends spam, why should domainB.com mail delivery on the same IP suffer from it?
Police the motherfuckingbejesus out of your users.
If it costs you when your IP is blocked, insist on a deposit that's forfeit in the event of spamming.
Sadly, I've worked for shops with poor practices. How bad? Sending thousands of emails to domains which no longer existed (let alone stale accounts at existing mail service providers). Simply not a priority.
