Hide blocks from particular users

Symptoms: There are blocks that you would like hidden from particular user roles (eg students); or blocks that you only want available for particular roles (eg for teachers).

Resolution: 

Go to the page with the block and:

• Turn on editing
• click the 'assign roles' icon in the block - and ignore the rude "you are not able to assign any roles here" message :-)
• go to Settings> Block: XXX > Permissions
• remove the roles from 'view block' as required