Hi Costas,
You can use a custom config file for the CKeditor by putting this in user.config
<add key="CKEditor:ConfigPath" value="~/ClientScript/ckeditor-mojoconfig.js"/>
and change to a custom file by saving the ckeditor-mojoconfig.js as ckeditor-mycustom.config
then make the config setting match your file name.
then in the toolbar declarations in the config file you can remove the items for 'SpelChecker' and 'Scayt' (SCAYT means spell check as you type)
Hope it helps,
Joe