Include a Luhn-style checksum or a custom hash as the last few characters of the key.

"Keygens are not just a copyright issue, they are a safety issue. Many embed droppers that deliver stealer malware or ransomware." — DevX

Here is a blog post exploring why "high quality" keygens are usually a myth and how you can actually get your hands on Cyrax in the latest games.

The Risks of Cyrax Key Generators: Why "High-Quality" Tools Are a Trap