When viewing the campaign that you would like to lock under Campaign Management, click on "Details". From there, you'll see the "Lock Campaign" button on the left-hand side of the page.

When you select the "Lock Campaign" button, you will be presented with the Lock Campaign dialog box:

Note: This dialog box allows you to set a message that you want potential editors of your campaign to see.
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Once your campaign is locked, you will see a bar appear under the tabs list towards the top of the page which states, "This campaign has been locked. You will not be able to make changes to items on this page:"

You'll then notice that this bar appears in every tab of the campaign, and removes any editable choice in any campaign or form that's part of the locked campaign:

Selecting the gray bar with the locked message on it revealed the alert message you've created, along with details of who locked it and the date and time when it was locked:

Any administrator is able to unlock the campaign by returning to the Campaign Details tab in Campaign Management. However, their action will be noted in the Lock History, which appears below the Campaign Details page:

Note: There is no way to password protect a campaign from any other administrator. There is no way to exclude administrators from a particular campaign.