You wrote: [...]
Yes, that is a little bit stickier of an issue, IFF your goal is to somehow continue to provide the would-be spammer with the ability to send traffic to the net, provided it doesn't transit your mail server. I feel that you're overlooking the simple solution. Blocking the entire account so they can't access anything is the proper response to a spamming incident.
If you block the entire account then the user can't use the account to download the updates your Abuse Team will responsibly want to point him/her at. If you want to lose the customer then that's your business. If you want to keep the customer, helping them fix their mistakes is probably a painful and thankless task - but important and useful to the whole Internet community. Regards, -- leo vegoda RIPE NCC Registration Services Manager