In 1992, Mt. Etna began erupting. The lava was threatening some important structures on Etna, so it was decided that the lava would try to be diverted from the town. Earthen barriers were constructed to keep the lava from flowing into the town. Concrete blocks were also dropped on the lava flows in attempt to divert the lava. The plan included blowing a hole in a lava tube at high elevations and then fill the lava tube with concrete blocks to stop the flow of lava in the tube. (See Figure below). This attempt was successful in sparing the towns, but some still debate if diverting lava flows at Etna by bombing a volcano is a good idea.
Earthen barriers attempt to divert lava flows from Etna in 1992. Source: Big Think
Concrete blocks dropped at Etna during 1992 eruption. Source: Claude Grandpey
Another example is Mauna Loa in Hawaii. Targeted bombing campaigns on vulnerable parts of a volcano, mainly spatter cones that are the source of lava tubes high on the slopes of Mauna Loa, might cause sufficient diversion as to prevent lava flows from reaching the town of Hilo on the big island (Lockwood and Togerson, 1980). The first attempt to bomb Mauna Loa in Hawaii was in 1935, some of the bombing was caught on video shown here. A second attempt in 1942, was done on Mauna Loa. Both attempts did not provide noticeable results.
In the 1970s, tests were performed by the U.S. Air Force and Hawaiian Volcano Observatory (HVO) scientists to see what might be the most effective means of bombing a volcano. These tests were on an older lava field with no active lava flows and were performed using relatively modern military supplies. It was found that if you target spatter cones that feed lava tube systems, then lava flow disruption occurred where the surface was most fragile and not dense, solid rocks like above some lava tubes. Bombing the actual lava tubes or flow levees didn't seem to be very effective (Lockwood and Togerson).
Bombing the lava flow to divert it, is a relatively low cost, especially if you look at the cost of lava flows reaching the populated area. The result of the bombing, if successful, is very effective, however, not all eruption of volcanoes will have vulnerable spots that are obvious to bomb if that was decided and could have unintended effects.