Got more WhatsApp spam so I'm here to say this again:
A great pattern to prevent spam is to require a stake to send a message which can optionally be slashed by the recipients (where the tokens are destroyed).

Required stake size can vary based on account reputation and other signals

The only problem is externality pricing.

Building https://negationgame.com

This approach effectively disincentivizes spamming while also providing a more nuanced solution by allowing stake size to vary based on account reputation.