I am going to strongly disagree with you, then scold you, and hopefully shame you.
They are most likely Collinista snails, not Astrea.
Other possibilities are Cerith or Nerite.
They are just a part of your cycle, if you keep the amount of crabs low they will always be in abundence. They are also self limiting. I personally have noticed that they either eat or keep corals free of cyanobacteria.