There's no way currently to get expedition crew to leave. It's best to wait until you have plenty of colonists available before building an expedition center.
Currently no way to move the expedition vessel list either, though we designed it so it never overlaps any expedition POIs.
Also, you can't avoid colonist deaths entirely in an ambush; risk is part of adventure. Your best bet is to keep the Expedition Ship fully staffed, your vessel in good shape, and research Zorium Bombs and related tech.