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. It's a long read with no real solutions.
Another site discussing the issue is junkfax.org where you can hire an attorney...
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 both expensive and time consuming.
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Transfer Your Existing Number
The best solution may be to get a new fax number. Since your number is probably published in several places, and your customers have been using it for years ‘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 from FAXtopia. 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 solution, but it’s an effective way to stop excessive fax spamming.