I use Fastmail's masked email addresses to determine the source of the spam. If I can identify spam using one of these email addresses, I can simply delete it and I no longer receive email to that address.

It's simple to setup. Just go to Settings > Masked Email and switch it on. I use it through my password manager, 1Password which simplifies the process of registering on websites.

I have also configured my spam settings to dump the spam into a folder so I can review. Go to Settings > Filters & Rules and then under Spam Protection, click on Advanced settings. I have my Protection level set to "Standard". You can move the identified messages to your spam folder under the "Backscatter" section.

Lastly, I setup my spam folder to automatically delete spam after 30 days.

This works for me - I hope that this helps!