You are able to block inbound numbers by selecting the Numbers tab on the left. From here you will select Blocked Numbers. From this menu you can manage or add a new number to your block list.
To add a new number you would select the Add button on the right.
Once you select the add button a new window will appear. From here you can input the number you would like to block. You are able to choose to block the number at the account level or the number level.
For example, many people report getting SPAM faxes from 200-300-4000. This is not actually a valid fax number. This is a spoofed callerID used by many (including the IRS). It can't be blocked.
Here is some info on junk faxes from the FCC that hopefully you will find helpful https://www.fcc.gov/consumers/guides/faqs-about-junk-faxes