Top-posting is an activity reviled by many on USENET, as being a violation of Netiquette. Sometimes responses to this can be quite heated. However, such anger simply invites retaliation in kind, which leads us nowhere.
Therefore, let me suggest an alternative approach: humour. Specifically, barbed satire in reply to a top-poster's posting, taking old jokes that have often been targeted (unfairly) against various ethnic groups, or (more legitimately) against various professions or other activities, and putting them to a better use. An example illustrates the basic form:
A: Skid-marks in front of the hedgehog.
Q: What's the difference between a dead hedgehog on the road, and a dead top-poster on the road?
This takes the form of a riddle, with the answer placed before the question, reflecting the backward nature of a top-poster's writing.
Some more examples:
A: Not enough sand.
Q: What do you have when a top-poster is buried up to his neck in sand?
A: You take off your shoes to jump on a trampoline.
Q: What's the difference between a top-poster and a trampoline?
A: Both have about a 1-in-3-million chance of becoming a human being.
Q: What do a top-poster and a sperm have in common?