Luckily you can’t because they are mandatory to have an valid user account. If you want to have account which can not login: then don’t create an account ;-).
My gut feeling tells me that you have an entity person/employee/customer or alike which you directly inherit from System.User or Administration.Account. Thats exactly the reason why you should model it using a association. See for more guidance this forum post https://forum.mendix.com/link/questions/88570 and this lecture https://gettingstarted.mendixcloud.com/link/path/16/Design-Advanced-Domain-Models
It cannot be done, since the System module is mendix generated module by default during the project creation. It’s a kind of default validation on this user object. For example, even if you add custom validation on Name attribute of User object during account creation, it will not effect in this case.