Yes this is normal, they are saying they are going to kill your (Red, huge warning at top of screen) ships because they are easy prey.
Solution:
Give them some big weapons (even disabled) or leave another ship with good weapons to keep it safe, just set it to patrol / attack enemies. No ships will spawn if you are out of sector except those, and as along as there is a ship in the sector that isn't showing as red warning they won't show up either. They want easy prey, not a fair fight.