If you want to completely disable the Spell Check As You Type (SCAYT) feature in CKEditor, remove the scayt plugin using the removePlugins configuration setting.
config.removePlugins = 'scayt';
If you want to leave SCAYT available, but prevent the feature from being turned on automatically on loading the editor, set the scayt_autoStartup configuration setting to false. This is the default value for CKEditor configuration.
config.scayt_autoStartup = false;