How do I kill messages based on content?

In Gravity try this.

Tools > Rules > Add > "Fred Bloggs" > Conditions > Article > DOES contain phrase "Fred Bloggs said"

or write the condition at the bottom box: Body contains "Fred Bloggs said"

However this traps all replies to Fred rather than just the threads you killfiled. You can killfile posts below a certain point in a thread by specifying the message ID because it is quoted in the References header in all subsequent replies. I don't think Gravity allows you to test the References header specifically but you could test for the message ID in all headers using:

Header contains "[message ID]"

By killing on the "References" header.

Get a life & stop whinging. Just ignore them.

Thanks, Pamela - I'll try that

As I've already killfiled Fred because I have no interest in the ramblings he posts, I've even less interest in any replies - in fact, it is these stupid replies that I want to stop!

Depending on your newsreader, it may be possible to killfile on the "References" header, which records the Message-IDs of all the previous postings in any thread. If your undesired poster has a sufficiently characteristic Message-ID (and you will have to determine this by inspection), you can killfile on it. Then, not only will it killfile the target individual, but any thread in which they have participated.

I seem to recall on Gravity you have to explicitly run the "rule" you create (using a button on the toolbar).

Maybe somewhere in Options you can set that rule to run by default.