I've tried the demo of N13 and installed it on my local machine.
It seems that even though it has a captcha field, you can simply submit a comment without having anything filled in.
Is there a workaround for this? The last thing I'd want is spammers having a field day.
You might have the captcha setting disabled in the system options. Options
> System
> Comments
> Use image verification on comments form:
No
Change this to Yes
or ReCAPTCHA