Officially; FCC regulations require fax senders to include an opt-out method. Also, the number can be reported to the FCC, which usually doesn't help much.
The fix for spam faxes is to block the fax number in your mail client settings. You can Create a Filtering Rule for that fax number.
For example, when you receive a fax from FAXtopia, the Subject line reads:
'You have received a fax from 555-555-5555' (shows the actual fax number)
Within your e-mail client, set a Rule that any email that contains 555-555-5555 is automatically sent to the trash / deleted folder. You will never again see it in your inbox.
All faxes that are sent to you are processed through our main fax servers. They are all sent from the domain '@faxtopia.com'. If you get a spammy fax you CANNOT mark it as spam. In effect, you would be marking FAXtopia.com as spam. Then, all your faxes would get blocked.
Here's some instructions on white-listing domains at various e-mail providers:
White-list Email Addresses In Gmail:
1) Login to Gmail, click on the gear icon and select "Settings":
2) Select "Filters and blocked addresses":
3) Scroll past all your existing filters and select "Create a new filter":
4) Add the email address that you want to white-list to the "From" field.
In Outlook 2010 or newer:
Go to Junk > Junk E-mail Options > Safe Senders > Automatically add domains to the Safe Senders List.
How to White-list Email Address in Office 365:
Open the Exchange Admin Center.
Click on the Mail Flow drop down and select Rules.
Add a new rule for Bypass Spam Filtering.
From the Apply this rule if… ...
In the field to the right of the Check names button, type the email address you want to allow.
For other e-mail providers please check with your service provider.
Here's an article on how fax spam started.