If you want to disable autocorrect and/or spell checker, you should change this in Windows settings. Latest version of Skype is strongly integrated in Windows system.
So how can you turn off the spell check and autocorrect feature?
1. Move your mouse cursor to the top-right corner of the screen. This will bring up the Charms menu in your Windows 8.
2. Click on lowest gear icon - "Settings".
3. Click "Change PC Settings" link at the bottom of the screen.
4. On the "Control Panel" page, click "PC & Devices", then click "Typing".
5. Change the sliders for "Autocorrect misspelled words" and "Highlight misspelled words" to Off.
This works on Windows 8 (and higher). If you still use Windows 7, you can open Skype Options -> IM & SMS -> IM Settings.