I added shreya_chatterjee13 to yahoo and she send me thousands of mess from evil_mask** what to do. HELP?

Deleate her and Mark the messages as spam
Then report her to Yahoo.
There are ways for her to get around things like this like make another Name But Like I say if you dont know the person dont open the message.
But Yahoo Can do alot for you. If she is doing this Yahoo can get her ISP and take matters into there hands and help you.
Goodluck