I have been putting forward all manner of strange ideas along the lines of weird science versus the volcano. Personally, I think a dome would be the idea, they did it with Chernobyl.
We're mainly interested in stopping the ash cloud, not the eruption. So fit the cap with extractor fans and suck the ash off, funnel it down into the sea, job done. You could even use the heat the volcano is kicking out to power it.
We've got to be able to beat it with some ingenuity, it can't be that difficult.