"possible follow-up" threading

How do I turn off the mechanism that includes a message in a thread because
the message's subject matches? Perodically, I have a coincidental match of
subject, so I really only want to match on message ID.

It doesn't seem like the <TSUBJECTBEG> tag will do this for me; it simply
removes the "possibly follow-up" tag, but still threads matching subjects.

Thanks,
John