Or simply, if you host your own server and you know how to manage it, create a blacklist based on senders, IP, domain, subject or whatever you want and create a rule for RFC 550 reject message saying: "If you want to send me an unsolicited message, please pay to this LNRUL or LN address x amount of sats, including your email address."

Or if you have that contact on a webpage, put a paywall that will force them to pay first before filling the contact form. https://github.com/cryptoteun/awesome-lnbits