Login to your web dashboard and go to
Users > Visitors
Search for the visitor you wish to block by name or email
Click on the visitors name to show their profile
On the visitors profile where you will see their visitor history and other options there is a menu in the header "Allow Sign-in".
Click the Allow Sign-in drop down
Select BLOCK to block them for sign-in at your locations
When a blocked visitors attempts to sign-in, the kiosk will alert them this is no longer possible and will alert a default contact of the attempt.