Restrict users to only one list

I haven’t tried it myself, but it would make sense to:

Create a form and include all existing lists.

Publish that form somewhere (anywhere) and copy the code for the checkboxes.

Create a new form and add Random Text or HTML.

Paste the code you copied, and change the checkboxes to radio buttons, thereby limiting the user’s selections — choosing one radio button turns off the others and vice versa.

Again, I haven’t tried it, but it sure seems like it would make sense.

Die Bombe, simply create just one list in MailPoet.

As for your WordPress users, they’ll always be in the list “WordPress users”.