How to disable "Assignable" checkbox when provisioning roles

Summary

Some roles default to "Assignable" when they're provisioned to users - we want to disable that

Content

Many of the SCM roles (and the custom versions we've copied from the seeded roles) are defaulting to "Assignable" when adding them to a user account. We want the default value of the "Assignable" checkbox to be not checked, so the IT team doesn't accidentally forgot to un-check it. How can this be done? 

Version

21A