"Blowing a ship full of Semtex up does not solve anything" - well it does, it means that they have "decommissioned" that Semtex, and have put the ball back into the other fella's court.

Of course they can perfectly well get some more, just as if they decommissioned some guns by melting them down they could always get some more. It's pure theatre, and a waste of time.

Where blowing up Semtex has it over destroying guns is that destroying guns has more overtones of surrender. Controlled explosions in a quarry somewhere maybe? Well, they could be seen as firework displays for the millennium, evidence of continued ability to wage war - but also they'd serve as a way of fulfilling the demand for a symbolic act of "decommissioning." Ambiguity is the key to successful negiotiation sometimes.