First, I know what you're thinking. Why do this? Well, we have a one-a-year project in which students must take a spelling test on the computer, but spell checking must be disabled. The computers are running 10.4, so your mileage will vary with 10.5. Since we could not find a good text editor solution that was familiar and easy on the eyes for K-12 students, I decided to look into the situation. In the end, we wound up using the bundled TextEdit, but I needed to figure out how to disable the underlined miss-pelled (yes, I did that on purpose) words, and prevent the user from running spell check at all.

To disable the underlines, I found a program called Despeller, which removes the underline by replacing the red-dot-line image with a blank image (note that this change affects every program, as it's a system-level change).

To prevent students from using TextEdit's spell check feature, I had to modify a .nib file within TextEdit.app itself.

First, making a copy of TextEdit. Take this copy, Control-click on it, and choose Show Package Contents from the pop-up menu. Then navigate into Contents » Resources » English.lproj. There you will find Edit.nib.

Depending on what OS you are running, this may appear as a folder, or you may have to do another Show Package Contents drill-down on it. Either way, once you are inside, look for the keyedobjects.nib file, and open that with an editor that can read .nib files -- I used TextWranger, but to each his own.

In the editor, look for these lines:

  <string>Check Spelling</string>
  <string>submenuAction:</string>

Change the second line so that the section now looks like this:

  <string>Check Spelling</string>
  <string>v</string>

Save your changes and close the file. Open your modified TextEdit.app, and you should be good to go.

[robg adds: In 10.5, the strings appear to be completely different, so I've marked this hint as 10.4 Only for now. If you have a method that works in 10.5, feel free to post it in the comments.]