What is the Shar'i evidence for the prohibition of some widespread customs, such as: collecting a sum of money from seven people named "Muhammad" and taking it to a blacksmith to make a "hazaza" to hang on the clothes of a child suffering from colic, and a woman in post-natal bleeding not entering upon a menstruating woman for fear of preventing conception, and young circumcised girls not entering upon each other?

The superstitions mentioned in the question are not supported by either Islamic law or reason; rather, they are forbidden. The first falls under the ruling of "amulets," and the second and third under the ruling of "superstition" (al-ṭiyarah), meaning "pessimism," and both are polytheism (shirk). Amulets are objects hung with the belief that they bring benefit or ward off harm. Whoever believes they are a means to bring benefit and ward off harm, while also believing that benefit and harm are in the hand of Allah, has fallen into minor polytheism (shirk asghar). And whoever believes they inherently benefit and harm has fallen into major polytheism (shirk akbar). As for "al-ṭiyarah," it is pessimism, and it has been established that al-ṭiyarah is polytheism.