.undef
Calling .undef with the name of a directive removes that directive from use.
If the given directive is not known, nothing is done.
Example -- Preventing system calls
The following example un-defines the system directive, .$.
.undef{$}
Example -- Ruining user experience
The .undef directive can be used to remove all directives from availability.
.foreach{d}{.known-directives}:
.undef{!d}