There is a simple way of deciding the problem - refill the Skype account. All ads are removed when you restart skype.
Microsoft (owns Skype) does not show ads to people who have money in the account.
You can't refill the balance less than $5 via Skype website. But there are other ways to put money on the Skype balance, without limit of the minimum amount.
Just refill Skype balance for 5 cents to forget about ads in Skype forever!
Sincerely, expert Brian.