Steve33 Flarum's permission system is additive. You will have to design your permissions the other way around: give a group to users who are not restricted. The group can be made invisible. Using additional extensions, it should be possible to assign this group immediately at sign up for new users if you wish.
Negative group permissions could be implemented using the extension API, but I don't know any extension that does this.
Depending on your use case, maybe https://discuss.flarum.org/d/27555-shadow-ban could be used. You don't get notifications for new discussions/posts, but you have a system to manually un-hide the new content created by the shadow banned users.
