I heard on the radio the other day that Yahoo has just experienced another glut of problems from the hack that took place to over a billion (YES it said Billion) accounts back in 2013.
https://www.nytimes.com/2016/12/14/t...hack.html?_r=0
There was just another hack recently.
http://bgr.com/2017/02/16/yahoo-says...your-password/
So you can either put up with the spam or block all yahoo accounts and tell your friends that you have blocked all yahoo accounts due to all the spam and if they need to communicate with you they need to find an alternative source. Quite honestly, in light of all the security breaches I would not have a yahoo account.