how do i politely tell someone to stop messaging me

Report that person to the moderators using lichess.org/report. It's possible that they advertised to someone else.

Tell him : 'This Conversation Is Blocked' and block the conversation

just let him have ataste of his own medicine rickroll him

rickroll @vellarco

already rickrolled him

@vellarco said in #15:
> already rickrolled him
pls can u send picture i want to see his reaction

Or just say:
"No."
And block him.

Block them

@vellarco Say: Please stop messaging me, it’s annoying and I’ll report you if you don’t stop. Also, I’ll join if you join mine, it’s that blue link I sent 😏