I use k2 user plugin and also user profile plugin together. In profile plugin, I choose Term of Service as required field and attach an TOS article with it. In the user registration form, I test it with choosing no on TOS, but to my surprise the user is created. This is going to be a problem for user who accidently choose no on TOS, but later on want to resubmit the user one more time.

Which K2 user plugin are you using?

Please see the attached picture for User - k2 and User - Profile I used.

Please ignore this one since it's working as expected.