I get it, but there's a double standard when it comes to so-called collective punishment. It's OK to impose sanctions on South Africa for apartheid but it's not OK to impose them on Gaza--or for that matter Iraq.
Maybe it's not a good strategy; it certainly doesn't deliver fast results: The UN first voted for sanctions against SA in 1962. Of course, many western nations talked a good game when it came to opposing apartheid, but they continued trading with her. And the whole Iraqi Oil for Food thing undermined the sanctions imposed on Iraq after Gulf War I. And that's before we discovered how corrupt the program was.
I should say that it's not a good strategy if it's not effectively deployed. If the UN imposes sanctions on South Africa and no one follows the sanctions, not much is going to happen. And if the UN imposes sanctions on Iraq and then gives Saddam a loophole you could drive a truck through, Saddam will remain in power.
But the Union blockade of the Confederacy worked very well. Probably because Lincoln didn't make an exception for peace activists from the North to breach the blockade.
Collective punishment--whether it be in the form of blockades, embargoes, boycotts or sieges--is supposed to bleed a population--or a leader or a party--white. So that they cry uncle and agree to stop doing whatever it is you want them to stop doing. Allowing "peace flotillas" to enter and dispense coriander and flower pots sounds great--harmless really--but it undermines the mission. Israel is basically at war with Hamas and the blockade is a form of economic warfare.