He would annoy the AIVD (or RIVD?), at some point possibly breaking a law (the threat). The AIVD isn't authorised to arrest I think, so they would make a complaint and a police officer would come to his door, interrogate him, recognise the mental health issues and send a note to his physician.

The physician would call the guy and try to get him into a voluntary institution or at least to see some kind of therapist on a weekly basis. Let's say the guy is unreceptive to his.

Next step is that a social worker in his neighbourhood (this is a new system called wijkteam) is assigned to him that will visit his residence once a week. The worker will knock on his door and try to connect and build trust, family and friends are encouraged to participate in this process.

If the guy screws up too bad though, and the police gets involved another time or two the physician will be asked to authorise an involuntary treatment and he would end up in an institution of which would be much like a rich state in the US (NY, CA) one I think.

Note that nowhere in this process there would be a solitary confinement or separation from his family. That might happen in the NL but I can't imagine it would ever happen for more than a few days.