I full second Michael Kazlow's comment about forcing an answer before continuing to the next question. In fact, unless the next question really depends on the answer of the question, it is rather bad practice to not let the user out of a question.
It is better practice to not allow for subsequent actions (such as submitting data) if there are unanswered questions. But then, even saving should not be prevented if the answers are not complete; it is OK to display a warning, but that's sufficient.
Also, if there are reasons to force the user to answer before continuing, there must be the option "no answer" or "pause" or "not applicable", allowing the user to leave the field. What exactly should be done depends on the result of a careful analysis of the process represented by the form.