Hi Mark,
You'd like to delete the meeting replies but keep the replies with additional text, right?
Acording to my research and test, I'm afraid that there is not a build-in rule for this. However you can refer to the suggestion below as workaround:
When you are composing a rule, you can see the window shown below. In this window, you can select "except if the body contains specific words" and ask your colleagues to add "Hi Mark" in the additional text. You can also select "except
if it is marked as importance" and ask your colleagues to mark the replies as importance if there is additional text.

Hope this can be helpful.
Regards,
Aidan Wang
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