Locking text boxes helps prevent certain content in your document from being accidentally or intentionally altered. To lock your text boxes in Word:
Locking text boxes in Word
Let's say you're working on a document that other people have access to on the same network, and you want to lock your text boxes. They're not accidentally edited.
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For this to work, you must use the tools provided on the Developer tab. The Developer tab is hidden by default. Activate the Ribbon tab if you have not already done so.
If you have enabled the Developer tab, open your document with the text boxes you want to lock and switch to the Developer tab. In the section "Protect", select the option "Restrict processing".
Note: The "Restrict editing" option is also available on the Review Available tab if you have enabled the Developer tab.
On the right side, a Restrict Editing pane appears, where you can restrict editing permissions for all or certain parts of the document. Check the box next to "Allow only this type of editing in the document" in the "Editing Limitations" section, then select "No Changes (Read Only)" from the drop-down menu in the same section. 
Next, you must select the entire contents of your document except for the text boxes you want to lock. The easiest way to do this is to press Ctrl + A to select everything in the document, and then hold down the Ctrl key while clicking each checkbox to remove it from the selection.
After selecting the content, check the box next to Everyone under Exceptions. This allows all users to edit the selected content.
Finally, select "Force Yes, Start Protection" at the bottom of the "Restrict Processing" section.
After the selection, you will be asked to enter a password for additional protection, enter a password and click "OK."
You will notice that all content except the text fields is now marked, so your text fields can no longer be edited The highlighted content may be.
To remove the document's protection restrictions, click Stop Protection at the bottom of the "Restrict Editing" section.