Is there an easy way to unsubscribe from a "Sign in with Apple" login
This is documented in https://support.apple.com/en-us/HT210426
On an iPhone, in the Settings app, under Apple ID -&gt; Password &amp; Security -&gt; Apps Using Apple ID, you can find this menu, which will allow you to disable forwarding from the private relay service. 
On a computer, you can access these same settings at the Apple ID website.

Click on &quot;Manage apps &amp; websites&quot; within the Securtity part
Click on the website/app you want to disable
Click on &quot;Turn Off&quot; (to prevent mails) or &quot;Stop using AppleID&quot; (to delete it)