Ask HN: How to implement 'free trial' that is acceptable to Google AdWords?

Our company offers Windows and Mac desktop software that includes a free 30-day trial period.

In order to prevent the user from continuing the trial after the trial period expires, the software writes the installation date to a 'hidden' place on the computer. Crucially, this date is not deleted when the user uninstalls the software. This prevents the user from repeatedly trialing the software without paying for it.

Google's new AdWords policy [1] no longer allows this type of behaviour. Specifically, "Once an application is disabled or deleted, it may not leave any remnants". Google support have confirmed that our software behaviour breaches this clause.

So my question is: How should I modify the software to offer a free trial without breaking the AdWords rules?

So far I see two options: (1) Simply hope that the user does not discover that they can extend their trial forever by continually re-installing the software; or (2) Have the installation program send some unique hardware ID to an online server to authorise the trial. Which of these options would be better? Is there a third option I'm missing?

[1] https://support.google.com/adwordspolicy/answer/50423?hl=en-GB&utm_source=policy&utm_medium=email&utm_campaign=gen