Go to the BETTER MTD console and select the Policy tab and choose a policy you want to edit.
Got to the network threat section and select Forbidden WiFi Connection.
Go to the Blacklist section and set your rule and add the SSID of the network you want to blacklist.
Go to the policy tab of BETTER MTD Console and select the network threats section.
Go to the Forbidden WiFi Connection section and select the blacklist tab. Change the rule and remove the blacklisted network SSID.