I guess part of the problem is that there are always some hostages being held whose lives can be threatened if any major military action is taken. On the other hand, maybe no one wants to spend the money and face the various risks incurred in launching such an operation. How about sending out some nice juicy-looking Q-ships (with lots of hidden firepower waiting on board) and laying traps for the pirates? That might discourage them some. If the odds of not returning from a piracy mission went up steeply, it would become a less popular occupation.