See, you can't do that. But I will show you a trick that will allow you to basically clear out a post.
First use the BBcode color=white, and then put a period inside it, like this:
[color=white].[/color]
You can use this to basically wipe out your post without actually deleting it.
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^-- See? It's totally invisible.
You can also try it with color=transparent too I think:
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Or use a very tiny font size like size=1pt for the dor. And you can combine these two methods too, to make a post that is basically empty by all accounts.