Obtaining plugin identifier of the plugins installed in the computers:

Let's consider a plugin, Adobe PDF reader as the plugin to be whitelisted/blacklisted.

Go to Internet Explorer.
Click on the "tools" icon at the top right corner of the menu bar.
Select "Manage add-ons" from the drop down list.
From the "Show" drop down list, select "All add-ons".
Click on the plugin to be blacklisted/whitelisted from the list of plugins.

How to check plugin identifier of plugins for Internet Explorer.

Click on the "Copy" button on the "More information" box.
Paste the copied content in notepad.
Copy the "Class ID" between { } and paste it in the "Plugin name or plugin identifier field" present in the Manage Add-ons page.