How do I disable or enable the MailScanner cPanel front-end for cPanel user accounts?

To configure the cPanel MailScanner front-end so you can enable or disable it for individual cPanel user accounts, go to WHM > Plugins > ConfigServer MailScanner FE > Front-End Settings > End User Settings > Default Settings (dropdown) and set "Allow access to the user front-end" to No. Click Save Settings. Then you can select specific accounts from the dropdown list and set "Allow access to the user front-end" to yes for  those accounts you want to have MSFE. 

Important: Both disabled and enabled accounts are set to the default settings under ConfigServer MailScanner FE > Front-end Settings  as well as the Advanced Settings (at the bottom), including Default for spam scanning, default actions for high and low scoring spam, and default for virus scanning, etc.. So if you do not want accounts without the front-end to have any scanning at all, you should set the defaults accordingly.