There is not currently a feature to block inbound numbers. If you can verify an actual sending number we can attempt to block it at the carrier level but if it's fax SPAM the number will likely not be valid.
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