In response to the statement "When is violence better than non-violence? When there are no other options." I think it's a little more complex than that, so here are my criteria - I reserve the right to add or subtract from this list as new issues arise:
1) There is absolutely no chance that any human being or animal will be physically harmed by the action.
2) The environmental harm caused by the action is less (cumulatively?) than the environmental harm caused by the activity to be stopped.
3) The action will not bring about massive resentment or antipathy from the local community or from the general public.
4) The action will effectively and permanently stop the activities.
5) There is no other option.
I don't know if there are any circumstances that meet these criteria, but I'm willing to entertain the possibility that there are.