To block ads on the skype application (tested with 7.12 from 28.10.15) , you can
add the following lines to your host (file:
%windir%\system32\drivers\etc\hosts) as administrator!
If you also add the following line, as stated in some tutorials:
the skype home can not be loaded any more.
If you also want to remove the Ad Placeholder, you have to remove the AdvertPlaceholder line from the config.xml file in the directory
%defaultuser%/appdata/roaming/Skype/(your skype username)/.
This can be done without Admin priviledges. After saving the effect is seen