How to tell users to post a message or their account will be deleted?

[setishock]

Seeing as there's little activity in your forum a heavy handed approach like that could be suicidal. I'd try a different wording.
(Sent out in a system wide email)
Hi,
Since you signed up you haven't posted anything. Is there a subject you want to discuss and didn't see where to post it? Or is there an issue with the forum itself?
Either way we'd like to have you back as an active member. However if you choose to continue not to post you leave us no choice.
On <for exsample> Sunday, 06/10/2012 at midnight local time, all zero (0) poster accounts will be purged from the forum. If you wish to continue to be a member you must come back and make a post of useful value. Otherwise your account will be included in the purge.
See you at the forum,
Admin

The lite handed method works a whole lot better.