I would not interfere unless it is obvious that the person will fail, in which case I would try to talk him/her out of it, just so that the person would not end up with permanent injuries or damage. I would never actively physically stop the person from CTB'ing, because that may make things worse, or it is possible that I may also get hurt in the process.
If I was a mandated reporter (teacher, counselor, professional, etc.), then I would just do what I need to legally do to avoid legal and professional trouble, and I would do everything I can first to avoid having to resort to intervention and only intervention as an last resort. By that I mean, like being direct and straightforward. For example (see below):
TAW122: As a mandated reporter (teacher, counselor, medical professional), if you are at risk of imminent harm to yourself, or others, then "legally" I am required to intervene, which includes notifying law enforcement, other medical professionals, and/or authority. I would however recommend making a safety contract to ensure that you will be safe.
Patient: I understand.
Depending on what the patient or person does, it will determine the next steps. If the patient still goes forward, even with giving him/her the hint at possibly breach of confidentiality due to the danger law, then I would be required to take action (even if I don't like it). If the patient retracts and plays it off, then I would only do what I need to do while meeting the legal criteria but also keeping the patients' interests at heart (it's very difficult and not many counselors and professionals do that).
Mind you, I would never intend to be a mandated reporter as my career path is unlikely to go in professions that would make me a 'mandated reporter'. This is just a hypothetical scenario of what I may do if I am in that situation.