Malacca Straits is an international water way the security of which is currently undertaken by 3 countries, Indonesia, Malaysia and Singapore. Any blockade of the straits is against international law and UN convention, and will be opposed not only by above mentioned 3 countries but also countries whose trades depend on this water way.
The first reaction would be an emergency meeting at UN Security council, and if US is the one that block the straits it can veto any UN declaration. Most likely outcome would be a military bloc (outside of UN) headed by China with almost all of ASEAN countries except Philippines. China would likely have the consent to base their forces in ASEAN, with the objective to unblock the shipping in the straits.
Which mean PLA may send ground forces, air force and navy to ASEAN, basically Malaysia and Indonesia, but keep the rocket force in China. How the war or battles are to be fought depends on what the opponent deploys. that is anyone's imaginations.
And this would make ASEAN abandon its neutral stand and will be ally to China.