Technically and theoretically we could reduce DM spam by using only paid relays. So the spam accounts would have to pay sats to be allowed to connect to those relays that you use to receive DMs. In Amethyst you can do that already and choose from which relays you want to receive DMs.

But right now you would limit your connectivity, as not all real accounts use the same paid relays as you do... 🤷‍♂️