Indonesia claims Malacca strait as a territorial sea.
"It is one of the busiest ocean highways in the world. An estimated more than 41,000 ships pass through the South China Sea each year - more than double the number that crosses the Suez Canal and nearly treble the number of ships that use the Panama Canal.
Its closure could generate a massive increase in freight rates worldwide and hit bulk shipments hardest."
In short, global economic disaster could ocurr if the strait was closed.
Indonesia can "claim" in one hand and crap in the other and see which one fills up first. The Straits of Malacca are considered an International Strait and the world's maritime nations (led by us) would never allow them to be closed by simple fiat.