A responsible spam protection system should allow every spam (and consequentially responsible user) from an ISP.
If a ISP shows sign of abuse, then show Captcha or other system that will block some spam while also blocking some valid users. This is a evil-for-the-greater-good solution. Do not fool yourself into thinking this is a solution (i.e. without caveats)
Impacted users can complain to both the service provider (you) and their ISP. And that failing, switching their ISP (i.e. voting with their wallet --how that happens in a monopoly is another discussion)
Bottom line, if you show captcha for all users (even for ISPs that are now showing signs of spam) you are intentionally blocking some users for no good reason. And you are part of the problem. Sadly, this includes the US government as they blanket censor all their forms (from visa request to DMV visits) behind Google(R) captcha(tm) at all times.