[dec0dedab0de]

don't respond to their emails.

If you want to be extreme don't distribute it to them in the first place. Licenses do not come into effect until after distribution. So you could have a pay-to-download model that comes with a %100 discount if you're a lone developer or an organization with under X amount of revenue. You wouldn't be able to stop someone redistributing it after the fact, but you're not engaging.

[LtWorf]

Unfortunately now that everything is based on automated pipelines, something that doesn't integrate well is not so good.

Although at work we have a provider of proprietary software that has an APT repository where the URL includes a secret token, so they can track from where it's being accessed.