From Envirowiki

Eco-sabotage can be defined as radical activism that is conscientiously undertaken with aims to stop or frustrate some action that is believed to be harmful to the natural environment through illegal or non-public means. It has often been referred to as “monkey wrenching.”

The place of eco-sabotage within the range of possible manifestations for activist dissent can generally be considered to be on the extreme, radical end. This has the potential to make it harder to justify in a moral sense than other forms of protest. It also makes political and public opinion outcomes harder to predict - in some instances, with good timing etc. it may have exactly the desired outcome. In other instances it may provoke backlash, and set the campaign back significantly.

Earth First! in North American were, and still are, to some extent, strong proponents of Monkey Wrenching. Their Logo included a Stilson Wrench, or Monkey Wrench, crossed with a rose, to this effect.