I've already installed a sign saying "No advertising materials/ No spam mail" on my mail box and continue to get spammed with them
Most notably spam from real estate agents
What's the best way to sign them up to spam to teach them a lesson?
I've got their mobiles and emails
not worth spending any time on this.
Spammers usually do not monitor the mailbox they use to spam others.
If you have full control over your mailbox domain, use a spam filter. Or, third-party service clean.email to unsubscribe automatically. However, I do not recommend giving third-party service access to your mailbox.