I dunno if yahoo operates an op system - could be wrong
but there are dozens of lamer tools out there to do that kinda thing - just go to google
but your best bet is to contact yahoo and inform them of the persons handle, time, date and room in which it happened -
the guy will prolly lose his account but more importantly they can check their records to see how it was done and maybe do something to stop it happening again - tho i doubt they will bother but u can but try