Hi,
I have joomla 2.5 with the k2 component version 2.6. I can not publish any comment.
In the configuration parameters've enabled the comments for all users.
here is the error that I get when I publish any comment on any article k2.

{"message":"Configura i tuoi parametri antispam nelle tab 'Commenti' e 'Avanzato' all'interno dei parametri nel backend di K2."}

This message translates to "Configure your anti-spam parameters in the tab 'Comments' and 'Advanced' within the parameters of K2 in the backend"

So, have you properly configured your antispam settings?

I think I've configured everything correctly.
I'll send you 2 settings screens.
It does not work that is enabling disabling the captcha