My b2evolution Version: Not Entered

Hi all,

first of all sorry for my bad English: I am a Bavarian!

I am using Version 3.3.1 and want to install a simple possibility to avoid comment spam: Only if the author of a comment click the "preview"-button it will be able to send the comment and publish it.
I suppose there are some lines of PHP in comment_post.php necessary? For example to remove the "submit"-button and only to show the "preview"-button in the first step. And after showing the review in the second step to show "submit"? Or so...

If someone - of you could help me, I will be very grateful an hope my question has been asked correctly. Many thanks!

Yours, Karin