I want to know the answer to the following question:
What forbid harmful  incantations?
All the background I'm provided with is that "The barrier between private religious practices and "magic" is permeable, and Ovid gives a vivid account of rites at the fringes of the public Feralia festival that are indistinguishable from magic: an old woman squats among a circle of younger women, sews up a fish-head, smears it with pitch, then pierces and roasts it to "bind hostile tongues to silence".". Is that enough to answer the question?
Answer: no