Hiding blocks from certain users

From Moodle 1.9 there is a [[Capabilities/moodle/block:view] moodle/block:view] capability which allows control of blocks based on the roles system.

Example: Hiding course block from guests

Login as a user who has permission to control blocks on the page you are interested in (e.g. teacher/admin)
At the top of every block there should be a face-and-mask icon, the mouseover tooltop says "assign roles", click this icon on the top of the course block
Click on "override roles" to switch to viewing role overrides
Choose the "Guest" role from the list of roles
Change the [[Capabilities/moodle/block:view]View Block] permission to prevent
Save changes
Guests should no longer be able to see the course block