Please create an additional field at registration

Help create an additional field in the registration of users, too many boats registered, and I do not have time to delete all, because you have to delete each one by one. Please help create a field in the registration, under the captcha, such as that of the equation was mathematical 2 +2 = and a field in which to place the only true solution (in this case 4), otherwise the registration is not complete. Someone versed in javascript help to do this, I do not have time to delete hundreds of boats at a time.
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https://github.com/vanillaforums/Garden/issues