In the world of faxing, there is no real spam filter. Fax technology is old….it does not have the same capabilities as modern e-mail. Once your number is on a fax spam list it can be sold over and over again on the dark web to endless new spammers.
Here's the laws and regulations from the FCC
Wikipedia has a good page about unsolicited faxes and the attempts to deal with it.
Another site discussing the issue is junkfax.org
Our fax servers block many countries including Russia, most of eastern Europe, Iran, N Korea and other known spammers, but that is not a full solution. The professional spammers use proxy servers that mask their real origin. A fax can be sent from overseas and appear that it’s coming from your own area code.
Here's one thing you can do:
If the fax spam is coming from a particular company and has a contact phone number, call and ask to be removed from their list. You can threaten to report them to the FCC. If your fax spam is random junk, then there is not much you can do about those. Since all faxes are delivered by your fax service provider you can't block them or all your faxes would stop.
So, the best solution may be to get a new fax number. Of course, your number is probably published in several places, and your customers have been using it for years. So ‘changing’ your number would be a big project. The clean way to resolve this is to keep the old fax # and then also get a new number. Over time, as your website and marketing materials are updated with the new number, you can eventually delete the old number. The spam will end.
This is not an easy answer, but it’s really the best way to resolve a problematic fax number.