R. Hershel Schachter spoke this Sunday on the subject of "Should I Call the Police? Clarifying the Issurim of Mesira and Chilul Hashem". Basically, he had said that anyone who poses a physical danger and will be treated fairly in jail should be reported to the police.
Regarding publicizing these issues in newspapers and on blogs when communal leaders (specifically rabbis) are not responding, R. Schachter said (link (MP3) -- question starting at 28 minutes 26 seconds, answer starting at 29 minutes, 27 seconds):
Just a comment on how come other rabbanim don't take a stand, I don't think it's a machloikes (debate). If you have a she'elah (question) on basar be-chalav (meat and milk), most rabbanim (rabbis) will not take a stand on it either because they don't know Yoreh De'ah (i.e. the relevant laws). If you have a din Torah (civil case) most rabbanim will not judge a din Torah because they are not familiar with that part of Shulchan Arukh. Most rebbes in yeshivos are... they have their noses in the Gemara; they don't have their noses in the Shulchan Arukh. I don't think it's a machloikes. If the other rabbis would look in Shulchan Arukh, they would do the same as Rabbi Gifter (who allowed reporting abusers to the Police).Note that at 32 minutes, 22 seconds R. Schachter discusses how one must ask a very specific question to an halakhic authority before acting on the above.
Look, if the thing is not taken care of and I am convinced that it is absolutely true, so I have no choice; I have to start a crusade and save the public from this person who is serving as a menace. I have to do something.