I think there is no way to do that, because additional islands will ALWAYS spawn, no matter how close they are to each other.
But anyways, if you are okay with that, I suggest changing Biome Size to great values, so the Ocean biome will be almost endless (and some islands will be large). But beware, you will need a /locate command for searching nearby land, if you spawned in the ocean.
Also, upping Sea Level will work.
This reply was modified 2 months, 2 weeks ago by hooves. Reason: addition