To prevent Word from spell-checking just one word in the document,
one could apply a style that doesn't spell-check.
This will require creating a new style, if you don't wish to destroy an existing one.
The steps to follow are :
- Apply a style to the terms you want skipped. You can use an existing
style or create a custom style but the style you apply to the terms
you want skipped must be different from the other styles used
throughout the document.
- Click the Home tab and click the Styles group's dialog launcher.
- In the Styles task pane, select the style you want skipped.
- From the style's dropdown menu, choose
Modify.
- In the resulting dialog box, choose
Language from the Format dropdown.
- Check the
Do Not Check Spelling Or Grammar option.
- Click OK twice and close the Styles task pane.

For more information see :
Skipping text in Word while spell checking a document.