You can do many things to reduce the negative effects of volcanic eruptions. The most obvious is not to build towns around a volcano even if the soil around volcanoes are fertile as volcanic eruptions do more harm than good to you. Spraying oncoming lava flow with seawater also helps. This method was proven when it was done in Iceland during an eruption in 1983.
The government can sent the military to divert the lava flow by bombing at the correct angle and place. These people must be trained and experienced or it might cause more harm than before. You could also use barrier walls to block and penetrate the flow of lava and use diversionary lava channels, so as to change the directions of the lava flow.
Educate the community about volcanoes, especially focusing on those communities most likely to be affected by the volcanic eruptions. If you are someone who lives near a volcano take extra precautions such as evacuation drills, also take into hand that emergency food and water supplies should be put in the same place so that when there is a need to evacuate it will not take a long time. Do remember that there must be an evacuation plan!
You can drop large concrete blocks into the tubes feeding the lava so as to divert the flow. You can also cool lava by breaking down the roof and sides of the tube feeding the lava flow. This however has never been tested as when workers went off to start this job, the town that was likely to be affected sent out military personnel to stop the workers.
You can also dig trenches away from nearby town/villages. Lastly, you can use Hazard Mapping and Satellite Monitoring, so you can tell exactly when the volcano will erupt. Thus, having enough time to evacuate the people from danger and death.