You can always drop a chunk of degenerate white dwarf into it.
If the mass of the target star + your bomb is greater than the Chandrasekhar limit it makes a pop that would startle some people. You would need at least a .4 solar mass object to do this.
Operation Giant Steelie
Procure a solid mass of iron .01 times the mass of the sun get it spinning until atoms at the equator are fixin on breaking free. This is a while past when they reach escape velocity. Gently lob it into the solar north pole. Because of conservation of angular momentum, this will cause the sun to flatten out and break apart.