If the captcha is only shown to delete posts, most Newbies will never even see it. Even the spammers won't be bothered by it, but it can end this paid bumping.
Maybe but I believe that captcha would be better when posting a comment AND deleting. Why?
-Well lower ranks need to wait a few minutes to post, therefore one captcha every 5 minutes isn't such a big deal for someone who's not spamming.
- How often do you delete posts? Let's say once a day and even if it's 2-3 times a day, quick captcha wouldn't be such a big hassle.
So with captchas on posting & deleting should slow down spamming significantly.