The Captcha won't stop them registering because typically it's people registering and then they hand the accounts off to bots who do the posts. But looking at the rate at which they post it may even be humans doing that as well.
The ignore function works only to a certain extent in that as far as I can tell, the ignoring of posts is implemented when the page is generated, not when the data is retrieved from the database. What that means is that even if you ignore a spammer, if they have filled the 200 (or whatever it is) new posts listed, you won't see any other new posts because the query returns 200 and then they are stripped out during the content generation. At least this is how it appears to work for me because I've ignored all the spam before and still not been able to see legitimate new posts. You will if they are intermingled with the spam but otherwise you won't.
