Although you can not turn off automatic updates in Skype you can however block any updates with your firewall settings. Locate your firewall go into program controls ( This will be a list of programs you have gave permission to at some point). Look through the list and change all entries you find for Skype setup.exe from allow to block. This is very easily done with any firewall that is worth its salt. Whenever your Skype receives the Skype setup.exe to update it will be blocked causing Skype to stop responding a simple reboot will fix this problem with no changes made to your Skype or system.
Please Note: You have already given Skype all the permissions you need for it to work if you should be notified by your firewall that Skype is trying to Connect to the Internet, it needs to remain blocked. If by some chance you should find this was a mistake you can go back to the same permissions list and allow it, it should have a date on the entry.
I have been told the last version of Skype is irreversible once it completes. This is not necessarily true. My Skype update on two separate occasions during startup ( before I made the changes in the firewall). Once booted I had to remove Skype completely with programs and features in the control panel. I then ran CCleaner Registry Cleaner to remove all registry entries from Skype (it is a free program easily found online). Once removed I had to use Windows Explorer to go into program data on local drive C and delete the Skype folder (very important). Once this was done I was able to reinstall an older version.
The latest version of Skype is no longer using H264 for video transfer. It is being replaced with the new Google format for video transfer. At this point I do not know how this will impact rolling back to an older version. Shame on Skype it was one of the seven wonders of the Internet. Hope this helps.