We've now added this to all RSS feeds by default. No setting or action is needed to enable this feature.
To verify that it is working, you can look at the "Podcast/RSS Feed URL" field for any subscriber, which you can find listed in their subscriber record. If you can view the raw XML for their Podcast/RSS Feed URL, you will find the <itunes:block> tag in the <channel> element of the feed. It will be set to "yes" for the RSS feed of all users by default.
The way to make an exception for the iTunes Block is when you manually add a user whose feed you WANT to be submitted to iTunes-- In which case, in the Add New Client window, you check to enable the option, "I will use this client's podcast URL to submit to iTunes".
In that case, the iTunes block tag will be set to "no".