How to prohibit invitations to external non-members?

It didn’t take long to solve this problem. I took a fourth look and discovered that sending invitations by email is only for external members, so removing this option does the job. Since every invitation, external or internal, is sent by email, I thought that “send by email” was for every kind of invitation. I suggest changing the wording, perhaps by adding the words “external to member directory” somewhere in the settings.