Hello!
I just have added a line to the CommonSettings.php (and deleted the individual lines at InitialseSettings.php) allowing each wiki to block logged in users.
Please note that whilst this feature is technically available, this doesn't mean you necessarily have the right to use it, because there might be policies on your wiki restricting this.
You can block a logged in user like you would block an IP, typing in an username instead of an IP.
Sincerly yours, Fire