The fact that the QlikView plugin for Internet Explorer is installed on a platform doesn't mean the user will forcibly use that particular client to open AP documents (unless all other clients have been disabled for that document).
To verify, navigate to the AP, click the Details link underneath a thumbnail and select "Internet Explorer Plugin" from the list of clients. If the IE-Plugin client isn't available, then you may need to reinstall it on that particular platform.
If you don't want anybody to be able to launch the AJAX client, you can disabled it in the QMC.
This is the menu from the AJAX client. Take a look on the available clients for this application within the access point by the link "view details". If the plugin isn't available it could mean that the installation wasn't successful.