This is how to disable it in the standard edition of Windows 8.1, for any other edition you might be better of manually locating the setting in the Control Panel.
Save the below as a .reg file and run it, the hotkey will be disabled immediately without the need for logging off. Alternatively, follow the instructions in the code comments to set it manually through the Control Panel:
Windows Registry Editor Version 5.00
; Note: This is for Windows 8.1 - the location of the settings/keys change with different Windows versions
; CONTROL PANEL:
; Control Panel->All Control Panel Items->Language->Advanced settings->Change Language Bar Hot Keys->Advanced Key Settings->Change Key Sequence...->"Not Assigned"
; WINDOWS REGISTRY:
; Primary Key: [HKEY_CURRENT_USER\Keyboard Layout\Toggle]
; Optional Secondary Key (might be needed for Windows logon screen): [HKEY_USERS\.DEFAULT\Keyboard Layout\Toggle]
; Values: "Language Hotkey" and "Layout Hotkey"
;1 = Key Sequence enabled; use LEFT ALT+SHIFT to switch between locales.
;2 = Key Sequence enabled; use CTRL+SHIFT to switch between locales.
;3 = Key Sequences disabled.