WRT1900AC

I have a WRT1900AC. I plugged a USB 3.0 external drive. If I enable secure folder access, I get two accounts: the admin and guest. the problem is that these accounts have access to the storage device. I want to disable the guest account and leave just admin. It doesnt seem to allow that. If this option is not possible, this router is poorly designed. Owners should be able to manage the users that have access to their storage. Inferior routers have this options.... Please help.