User Directory Id

Directory ID is per default equal to Identity, you should not change it, except if you are using an external Identity Provider (Azure).

How you prevent? Restrict the access to this form (or better to the complete Solution Manager) using Permission Sets, which you have to create.

More information: Users - Technical Documentation For IFS Cloud

BR

Directory ID can be changed, but kept unique for the area you want to use for logging in. 

If you let be the same as the IFS identity, then you’re planning to use the IFS IAM (Identity and Access Manager). If you want to use SSO (Single Sign On) with another area (i.e. Active Directory), the Directory ID represents the unique ID in that space (email address, or whatever is unique there).

I like IFS being this flexible.