yahoo messaging - constant messages from stranger... how do i stop?

[joko]

its getting on my nerves... i am forever getting boxes popping up with a message that someone wants to chat - someone called mamehandafxxx or sahhkateee or some such ***, i decline and block but how can i stop them appearing? i used to only get one a week now its 1-2-3 a day

is there a setting? i want to keep chat on for my mates.

thanks

[factor-fiction]

Yes I keep getting one- from the same person- every day. I ignore or decline them but they keep on coming