Uninstalling the add-on

1. If you wish to remove the 'Mail Merge' add-on, just go to your Google Document and click Add-ons->Manage add-ons...

2. Then click the green Manage button and click Remove from the menu. If the remove item is disabled or doesn't appear, it means that your administrator doesn't allow you to remove it however you can disassociate it from the opened Google Document by unchecking the 'Use in this document' menu item.