|I guess, this router does not support to close port 80 and port 443 for any other traffic than HTTP.|
Skype uses any TCP port from 1 to 65535.
If this is not possible, it tries to open a connection on port 80 (not HTTP), to connect to the Skype network.
If this is also not possible, it tries to connect to the Skype network on Port 443 (not HTTPS).
If this is also not possible, Skype is fully blocked.
So if your router can block any protocol except HTTP on port 80 and block any protocol except HTTPS on 443, you can block Skype. Otherwise, you can't.
I'm not aware of a SOHO router, that supports such feature.
The original poster should always write the last response !!!
Let us know, if the problem is solved !!!