There are 2 ways to Opt-Out of Freemius.
When you activate your plugin. You'll see a prompt like this:
Just click Skip to automatically opt-out of Freemius.
When you missed the first-activation prompt, you still can opt-out of Freemius by going through the Plugins > Installed Plugins page in your dashboard.
Find the Stackable - Gutenburg Blocks in your list of plugins, then click Opt Out.