how do I block certain email addresses from joining?

First of all I am not entirely sure if these are random spammers to recently join my website but I am unfamiliar with these email addresses...

@126.com and @163.com 

Is there a way where I can block certain email addresses from joining the website?

One other question was how is it possible that this could be happening if captcha is enabled on the join form?

Quote · 16 May 2012

How to restrict some of email providers on registration

Quote · 16 May 2012
 
 
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