I just learned with master pages, you can override items by pressing control, shift and left click the item. Or you can use this option on a page:
But how do you selectively prevent items from being override-able after it has been override?
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From the Adobe ID help:
To remove master overrides from one or more objects, select objects that were originally master items. In the Pages panel, target a spread and choose Remove Selected Local Overrides in the Pages panel menu.
To remove all master overrides from a spread, in the Pages panel, target the spread (or master spread) from which you want to remove all master overrides. Choose Edit >Deselect All to make sure that no objects are selected. In the Pages panel, choose Remove All Local Overrides in the Pages panel menu.
Note: if you've not over-ridden, but rather detached items, this will not work. If that is the case, you'll have to delete your detached items and re-apply the Master to those spreads.