‘ And Joshua charged them with an oath at that time, saying, “Cursed be the man before YHWH who rises up, and builds this city Jericho. He will lay the foundation of it with the loss of his firstborn, and with his youngest son he will set up the gates of it.” '
Having devoted everything to YHWH Joshua now devoted the mound itself to YHWH. He put on it a curse, that a city should not be rebuilt on it (Deuteronomy 13:16), in the strongest terms he could think of. The loss of a firstborn and of a youngest son were both seen as appalling tragedies, the former especially to a man, the latter to a woman. This later remarkably came to fruition over four hundred years later when someone did rebuild it (1 Kings 16:34). (This was unlikely to refer to a recognised sacrificial ritual otherwise it would not have been seen as unusual). Indeed Joshua may have intended it to be seen as signifying that the man's whole progeny would be destroyed one by one as the building progressed, from eldest to youngest.
Such a curse on a ‘devoted' city was seen as having great effect well beyond the bounds of Israel. The same happened to Troy and Carthage which were deliberately left desolate. It is ‘the wicked man' who ‘dwells in cities that have been cut off, in houses which no man will inhabit' (Job 15:28).
This does not mean that no one ever lived there, for settlement did possibly take place there (Judges 1:16; Judges 3:13 - although these may have been in tents at the oasis - Joshua 18:21; 2 Samuel 10:5; 1 Chronicles 19:5), but the idea was that it was not to be rebuilt as a city. (For the record New Testament Jericho was not situated on the old site).