If you need to disable spellchecker for CKEditor you can disable it with scayt_autoStartup setting. just set it to false and spellchecker will be disabled, but it will be not removed, so you can use it just clicking on spellchecker button.

<script type="text/javascript">
    CKEDITOR.replace('your_text_area_id',
    {
        height: 400,
        scayt_autoStartup:false
    });
</script>

Another solution is complitely disable spellchecker plugin for CKEditor, you can do it with setting removePlugins

<script type="text/javascript">
    CKEDITOR.replace('your_text_area_id',
    {
        height: 400,
        removePlugins = 'scayt'
    });
</script>

I believe the first solution is better, because you can use spellchecker if you need it, and prefer first solution in my projects.

How to remove spellchecker for CKEditor globally

This two solutions works only for one specific CKEditor, but if you need to remove spellchecker for all your projects you need to use config.js file. This configuration file contains all global settings for CKEditor.