What methods of protection against sheetposting exist for decentralized systems?

Question

CAPTCHA

Let's say there is some p2p chat, the participants of which communicate with each other.
The attacker automatically fills the chat with a huge pile of meaningless information, interfering with communication.
If it were a centralized chat, it would be enough just to add a captcha to this system, which would seriously complicate such a task.
And what to do in a decentralized system? Is there any cryptographic captcha that could stop the flow of meaningless information? How do such algorithms work?

Answer 1

"White"/"black" list of participants (ID/hash) on clients solves the problem instantly.

Answer 2

Improve the chat client so that several messages from one user are glued into one in the interface.
Long messages (including glued ones) collapse to a certain height.
For collapsed messages, show a counter of the number of real messages that fit into them.

Answer 3

Encryption of traffic between chat participants.