Trick is working great on yahoo messenger 9 version. I have tried this method and there is no yahoo ad on my messenger. It is a simple trick , the yahoo messenger pick the ad from this url insider.msg.yahoo.com . So, if you try the below trick the local host will block this url and no yahoo ad will appear on yahoo messenger interface. Its working good in yahoo messenger 9 version, havent tried on lower yahoo messenger versions.

Steps For Removing These Kind Of Ads From Yahoo! Messenger :-

* Close Yahoo! Messenger
* Go To " C:\WINDOWS\system32\drivers\etc "
* Open The File Named "hosts" In Notepad Or Any Other Editors
* After The Line
127.0.0.1 localhost
Add
127.0.0.1 insider.msg.yahoo.com
* Restart Windows.