∅, a slashed O. A symbol representing the empty set, or nothing.
The reversed symbol, 🚫, carries meanings of negation or forbiddance, as in "No Smoking" (🚭).
Examples:
post #3144998 Forwards (T-shirt with this is a signature for this character)
post #4536778 Backwards (She is forbidding it)

For null bulges with padlock icons, use padlock_symbol.
post #4393314 Similar meaning so often tagged on other symbols like this one, that means nullo or nullified (as in gender tag). Not to be confused with null.