In previous post, I described how you can add code to your VSTO/COM add-in to disable your Web Add-in loaded from maniferst via the Office Portal:
Removing Web Add-in Ribbon Customization in Outlook for Windows (theofficecontext.com)
Well with Outlook for Windows, Office 365, version 16.0.13728.36136, you can now disable your Web Add-in via Policy settings.
Here is the path in the policy editor for this setting:
Once in the policy editor, you would set the value. If your OfficeJS Web Add-in has a GUID of {c5bc7737-0d79-4302-9e73-2a614941e914} and the COM add-in is called “My Full Featured Outlook Add-in“, you would set the values like this:
Specifically, the registry key it puts in place is this:
PATH: HKCU\SOFTWARE\Policies\Microsoft\Office\16.0\Outlook\addins\equivalentcomaddin
KEY (string/REG_SZ): {OfficeJS GUID}
VALUE: Prog.ID
For example, if I have an Outlook Web Add-in called “My Outlook Add-in” and it has the following manifest entry:
<?xml version="1.0" encoding="UTF-8"?>
<OfficeApp
xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"
xmlns:mailappor="http://schemas.microsoft.com/office/mailappversionoverrides/1.0"
xsi:type="MailApp">
<Id>c5bc7737-0d79-4302-9e73-2a614941e914</Id>Let’s say I have a full VSTO add-in called “My Full Featured Outlook Add-in” and it is registered as the following:
If you were to look at the Policy key after it is set you would have an entry like this:
KEY: {c5bc7737-0d79-4302-9e73-2a614941e914}
VALUE: My Full Featured Outlook Add-in
If it were an older traditional COM Add-in, like the top item: AccessAddin.DC, then it would look like this:
KEY: {c5bc7737-0d79-4302-9e73-2a614941e914}
VALUE: AccessAddin.DC
That is all you have to do if you have the newest builds of Outlook for Windows.
NOTE: Coming later in the year, will be an option to add this entry to your Web Add-in Manifest file the way you can with Excel and Word.
NOTE: This is all covered in the latest Office add-ins community call.
While it resolves the issues of OLE initialization and allows you to edit your Excel embedding in Word (as one example), it does not prevent one other scenario that I like to call “Orphaned Excel.” In this scenario, you edit your embedded Excel instance in another Window (usually via a right-click / Ole Object / Open). If you leave Excel open, return to Word and close the document, Excel should close. But in the VSTO COM/OLE scenario it may not – it will remain open with the embedded workbook still editable. However, it is orphaned an no longer associated with its container. Any edits will be lost.
Your add-in is loading in the same AppDomain as everybody else. If there is another VSTO 3.5/4.0 Add-in loaded in Excel and that add-in is not doing any of the above – well, your still going to have problems. 
You can play by all the rules, but you cannot prevent other kids from throwing sand. This is why I see some customers going to the extreme to manage out (disable) all other COM add-ins when they load their solution. But this does not work for HKEY_LOCAL_MACHINE Loaded add-ins. 