I also associate shields with virus protection. Critical software updates, too.
Before LJ had their current cunning little double-lock (I love that thing; it's something I'd consider commissioning and wearing, even though I don't use the feature very much) or tags, I wanted to have a strong symbolic association for a filtered entry, so I borrowed from the UML class diagram visibility symbol for "protected" and have been marking filtered entries (or unfiltered in some cases where it's content that looks like it might deserve a filter) with the subject line including "# = [filter description]". Thus, an entry locked to my Support/volunteer-affiliated friends might have the subject "Another day, another person who has mistaken tech support for the contact-a-reporter department! ( # = spr0t )"
no subject
Before LJ had their current cunning little double-lock (I love that thing; it's something I'd consider commissioning and wearing, even though I don't use the feature very much) or tags, I wanted to have a strong symbolic association for a filtered entry, so I borrowed from the UML class diagram visibility symbol for "protected" and have been marking filtered entries (or unfiltered in some cases where it's content that looks like it might deserve a filter) with the subject line including "# = [filter description]". Thus, an entry locked to my Support/volunteer-affiliated friends might have the subject "Another day, another person who has mistaken tech support for the contact-a-reporter department! ( # = spr0t )"