Is it Islamically permissible to wear a necklace containing sacred writings to get rid of nightmares, and what should someone who is wearing it now do to get rid of it?

It is not permissible to go to soothsayers, fortune-tellers, and sorcerers. Whoever asks them about something, their prayer will not be accepted for forty days. And whoever believes them in what they claim has disbelieved. Similarly, it is not permissible to hang beads and amulets, and whoever hangs an amulet has committed shirk (polytheism). Some scholars have held that hanging amulets that are not from the Qur'an or Sunnah is shirk. As for amulets made from Jewish talismans, or by star worshippers, or from beads and strings, they are undoubtedly shirk. Ways to prevent nightmares and disturbing dreams can be found by referring to specialized sources.