Preventing account creation from certain IP ranges
Fragment of a discussion from Project:Support desk
Note, you can block a range of ips well still allowing logged in users to edit from that range but not allowing people to create new accounts from inside that range(but anons can't edit from that range)
If you can't find support in mediawiki for doing what you need to do, you can look into using/creating an extension that hooks into AbortNewAccount
