You can't, unfortunately. There are people who love testing new things and there are people who hate this, and for those who would like to test they simply don't give the chance. This is a server-side toggle, so, no... There's no possibility to disable a specific person.
As a premium user, I think users should choose if they want to be part of the tests or no as Netflix does.
Thanks for searching for the answer you need here!
As @Hubo mentioned, we're constantly running tests in order to provide you with the best music experience possible. So it's possible that you might see something on the app your friend doesn’t, or get a brand-new feature for only a temporary period.
Currently there isn't a way to opt out from such test or exclude specific accounts from our end However might we suggest creating a new account as a possible workaround? If the new one is not part of the test, you can transfer all your music there so that nothing gets lost. This page has detailed instructions on how to do this.
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“Music is a language that doesn’t speak in particular words. It speaks in emotions, and if it’s in the bones, it’s in the bones.” - Keith Richards