I think it is absurd that almost every time I hit the "preview button" to check how my post will look, I get flagged as spam, have to complete a captcha, and only THEN can I see the preview.
Out of a series of 10 previews and a single post command, I had to go through this 7 times!!! Just have the captcha there all the time if your spam is that bad!
I can't imagine that you have that much spam that you have to drive people away like this. This will make sure that you don't have spam, no doubt. Until all the people who are trying to post leave and all you are left with are the spammers.