You have to do two things to completely switch of the "Check Spelling" option:

1) remove the button "Check Spelling" from the following template: message.html, message_reply_write.html, post_edit.html, post_reply_write.html, post_write.html.

2) in your admin area, go to "Setup -> Actions -> Edit Actions" and then set the action "Spellcheck" to disabled.

Ivan
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Iyengar Yoga Resources / GT Plugins