Please forgive my ignorance, but why davka Yahoo?
If you need the net for work, why dont you use any other page (that has no access to chat) to gain internet access? Is internet chatting or using a site that has chatting part of your work requirements?
Perhaps you can start puting a fence by not going to any website that has chatting.
Another possibility would be to commit yourself (and ask Guard if a neder would be appropriate) to the following:
Prepare in writting, beforehand, a short statement stating something like,
"My friend, please be aware that while I am here in front of my computer writing to you, I am doing so against my will. I have been kidnaped. But please, do not call the police. I have not been kidnaped by a human being, but a dark and sinister impulse that resides within me. You see, we all have been bestowed with the precious gift of life. I myself come from a nation of prophets and sages who have taught the world how to lead a worthy and meaningful life. Regretably, for years on, I have squandered my life pursuing imaginary vanities and fleeting pleasures. I realize that while I sit here typing some nonesense, my life is sliping away. I have decided, however, now and forever, to make mends with my true self and with my Creator. I no longer desire to have any converstation, not a word, with you or anybody in a chat room and I request that you click me off your screen. I do not hold anything against you and I please ask that you forgive me if my chatting has led you to sin in the past. Thank you."
Loynata, if you feel that you have to chat, u can first post this. Yes, make it the first post. if you still have the urge, then you may .
Behatzlacha
ps u probably are aware that many "young girls" in chat rooms are perhaps men of your age
hatzlacha