The blacklist is used to prevent customers from purchasing from your instance by blocking them on the Order Import API call. Blacklisted customers do not reach the gateway.
...
Go to Admin and select Blacklist:
...
You can either import a file via CSV by clicking IMPORT FILE or add who you want blacklisted individually by clicking the + button.
...
When the + button is selected, a dialogue box will pop up which will allow you to specify what you want blacklisted; card bin, country/state, phone number, email address, IP address, and address.:
...