Is anyway can we allow/restrict user to force login to DSS with Upper or Lower case?
No, there is no restriction for creating nor using whole upper or lower case login.
Below option allows both Upper and Lower, if we uncheck profile create with first login which case we used.
Administration -> setting - > Login (LDAP, SSO) & Security> Other security settings -> Case-sensitive logins
When disabling this option, user login becomes case insensitive, means any case can therefore be used.
For instance "Admin" would be able to use "admin", "ADMIN" and any case combination possible to login.