If the add-in is already installed... log into the Home Server Console, select Settings, then the Add-ins tab and click the Uninstall button for the add-in you wish to remove.
If you are looking to get rid of the health warning that some Home Server's produce, informing you that an add-in exists... you can either click the Network Health icon in the console, locate the issue and check it's "Ignore this issue" checkbox... otherwise simply remove the offending MSI from your \\servername\Software\Add-Ins directory... in the case of McAffee I believe the name of the file is TopWHSaddin.msi.
Though when it comes to New Connector Available, you really should update to the latest version.