This advanced Spell surrounds their soul and it's signals, making their Soul Wavelength disappear and camouflages their soul like that of a normal Human. This Spell is capable enough of evading Soul Perception from a Humans and Death Gods. However, an individual is unable to use any of their innate Magic while under the spell's effect.
↑(Yen Press) Soul Eater Manga: Chapter 5 — Stein: Soul Protect. It's a high-level spell [Magic] that certain witches can use. They surround their soul with a spell, making their wavelengths completely disappear. The spell [Magic] camouflages their soul to seem like normal human souls.