It is true that VPN services which don't log any user activities attract
people with bad intentions and believe me: We are not happy about that
either...but we have to live with that
As per your own admission, you have to live with people abusing your service, but it doesn't mean others have the same obligation.
If you do serve people abusing your service by doing things warranting spamhaus listing as per they policy, why should spamhaus stop listing those IP addresses? By your own admission, the listing is correct.
It is up to you to come up with a solution that will stop your users abusing internet resources not belonging to you, and other intenet users are not obligated to accept such abuse.
As for extending the listing to cover the whole subnet, it is called escalation. Look it up, it is explained in spamhauses web page.
Yours,
esa
ps. to put it simply: you're entitled to send crap to the internet. Others are entitled to refuse receiving it.