disable guest accounts for commerce
I don't actually think Commerce has an option to disable guest checkout, no.  

But Craft itself makes this, in effect, quite easy.  (In fact, Commerce's whole concept of customers is largely separate from the Craft concept of users.  A customer may be a user, and vice versa, of course, but they are really two separate things.)

In any case, you can hide all checkout access and pages behind {% requireLogin %} tags so those parts of your front end are simply then not reachable by customers who are not logged in users.

https://docs.craftcms.com/v3/dev/tags/requirelogin.html