In some instances, a shared add-in that is listed on the Add-Ins tab in the Add-In Manager dialog box cannot be removed. The 'Delete this Add-In' button is disabled.
This issue occurs if the shared add-in is located in a well-known folder, which can be a network share. Users are not allowed to delete shared add-ins directly from the Add-In Manager dialog box.
Remove the shared add-in by deleting the file from the well-known folder.
\\network drive\Shared Add-Ins\