Is it permissible for those afflicted with thalassemia to refrain from procreation for fear of having an ill child, or is there a prohibition in that?

There is no harm in ceasing procreation, provided that the means adopted does not completely cut it off. One should not despair in seeking treatment. They should also frequently engage in supplication, remembrance (dhikr), and seeking forgiveness (istighfar), and utilize legitimate means such as ruqyah al-shar'iyyah (Islamic exorcision) and consulting specialists.