(Yes, that means you still need to keep your IE up to date, even if you use Chrome or Firefox.)
0) Exit Skype to unload the Internet Explorer ActiveX control.
1) Open Internet Explorer or Control Panel
2) Open Internet Options
3) Go to Security and select Restricted sites
4) Click Sites
5) Add https://apps.skype.com/ and click OK
6) Add http://apps.skype.com/ and click OK
7) Go to General, click Delete, and click Delete again
8) Open Skype, the blank space should be gone and there will be no ads
I don't know what the implications are and whether there any issues with other skype apps. Your milage may vary.
If you want to go further:
http://www.kgk.gr/2014/01/29/skype-ads/
Seriously, Microsoft, you make millions/billions and you still need 3 cents a click? Pitiful.
Oh, right I forgot, you decided to turn skype from a free peer to peer application that elected supernodes automatically into a server-sided monstrosity to support E911 calling. Bravo. *facepalm*
