[nmstoker]

Perhaps the client could help here? :-)

Other user types any of the common greetings alone (hi/hello/hey)

Client responds with: "this is an automated response - your greeting has been blocked until you provide a more meaningful reply"

Then filter out some rude/idiotic replies and keep filtering until the response's entropy is above a threshold and the sentence has some basic structure or presence of a couple of POS (eg a verb and noun)

[juancampa]

Snap! Great suggestion there :)