If by “lock” you mean to lock the position so that your text box moves with the text, you do that with the Text Wrapping dialog box.
On the Position tab there are two settings at the bottom: Move object with text and Lock anchor.
With these two options selected, this is what happens with my text box when I add more text to the preceding paragraphs.
In this example, I unselect Move object with text, but leave Lock anchor selected.
See the difference when the paragraph is moved down. The text box remains locked where it was on the page.
If, on the other hand, you want to lock the box and the text inside of it you can use the Restrict Editing feature. On the Review tab click on Restrict Editing.
If the Restrict Editing option does not appear, you may have to enable the display of the Developer tab.
Once you click on Restrict Editing, the Restrict Editing pane will appear.
First, press Ctrl+A to select the entire document, even if there isn’t much text yet. Now while leaving the document selected, Ctrl+click on the text box to unselect it.
Then, check the number 2 box – Editing restrictions and make sure that no changes are enabled.
In the Exceptions section click on Everyone. This will allow everyone to make changes to the area you just selected within the body of the document.
Now click on Yes, Start Enforcing Protection.
You will be prompted for a password, which is optional. (If you assign one, make sure you remember it or you will never to able to edit this textbox again!) Once you click on OK for the password, the Restrict Formatting and Editing pane will show the following options. You can unselect Highlight the regions I can edit so that the yellow markings disappear.
Now users will be able to edit the document but not touch the textbox.
Hope one of these answered your question.