If you are receiving faxes through an online service then you can't block the number. All your faxes are coming from the same fax server. The Caller ID from a spam sender is usually 'spoofed' and not the real sender. That type of spam is difficult to block.
For a large company, the local phone carrier can set up a 'Call Trap' on your line to find an individual sender. However, this is expensive and time consuming.
In the world of faxing, there is no real spam filter. Once your number is on a fax spam list it can be sold 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.
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Transfer Your Existing Number
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.