the main method against spam in Nostr is probably lightning. You can simply attach sats that are worth a couple cents to the content. Note that lightning has the concept of hold invoices so the receiver may or may not chose to actually claim the money. I.e. your friends and family will always attach 50cents to a message, knowing that you will never claim it anyway. If a spammer attaches 50cents to a message, the recipients will claim it and the spammer looses.

This is where we are headed in the AI age without Nostr/Bitcoin anyway. On some sites like youtube it has already become impossible to separate spam bots fom real comments. Agents that remote control users real browser instances and post AI slop all over the network will bring spam to another level. Attaching some money to whatever you post will actually be usefull to separate spam from real content.