Hi damien,
by default a new user should NEVER be able to set its own role.
This should be handled under the hood and/or be handled by an administrator.
In Mendix you can set the role of a user in a microflow, by retrieveing a specific role and assign it to a (new) account.
The Userrole select should never be available to a normal user.
As example see this model share (hint, download in your model)