If they really oppose the radicals, then there are many ways they could respond, from an anonymous phone call reporting where the radicals live, to a quiet knife in the back some dark night as the radical returns home
The radicals could not survive for long if there was a significant number of Muslims who knew who they were and were willing to do something about it
I'm afraid that the Muslim community can more realisticly be divided into "violent radicals" and "silent supporters", with the latter possibly disapproving of the tactics, but never enough to actually do something about it.