Hi James,
1. Go to Advanced Settings > Customizations > Customize the System, expand the Entities, click the entity that you want to remove from + button, deselect Allow quick create. Save and publish.


In this way, this entity will be removed from quick create entity list (the top + button beside advaned find) of all apps.
2. In addition, if you only want to remove some entities from the + button of an app, and other apps are unchanged, then you need to remove the entities from the app designer.
Go to Advanced Settings > My Apps, open the app designer.

Click Entities and deselect the entity that you want to remove form + button. Save and publish.

In this way, this entity will be removed from the + button in this app, other apps will not be changed.
BTW, you only need to choose one of these two solutions, not both.