This sounds like an issue outside of Thunderbird's control as it appears it is the email itself, not the Thunderbird client that has become the target. If they are using the same email addresses (or a common domain) create a filter that auto deletes (as opposed to sending it to trash or spam folder) for them all. A more effective and long term solution is to report the emails via your email provider that is receiving the spam (report as spam of course). Eventually, they will be shut down by larger companies/email providers.

First mistake is to opt out of something you did not sign up for. Spammers love it when you do because it confirms hey have an active email address.