Get a few companies to agree that open proxies are a scourge that needs to be stopped. They each apply some action to open proxies (user-facing messaging, loss of functionality, captcha, or complete block), and the users of those proxies will get the problem fixed.

The hard part (and it truly is hard!) is convincing a few companies to do this. It risks user complaints in the short term, to solve a problem that may not be very acute for the largest companies (who can simply absorb these attacks).