Just look at the Dok do Island; South Korean just sent the troops into the island, occupied the island and forget about Japanese complain.
When you deal with these Jap, you should just use force and occupied these islands. This island in question should be return back to China as part of the Japanese Surrender during WWII.
So, what right does these Japanese have to take this island?
If Japan don't return these island, it means they don't accepting their WWII surrender; thus, this is a declaration of war. This will allows the PLAN to blow out any Japanese ships in the area.