Pop one string representing the name of the property to be deleted. Then pop the object from which the property is to be deleted.
96 04 00 03 00 'a' 00 3A delete("a");
to delete a global variable.
According to some movies I have looked at, the delete instruction returns some undefined value. In newer versions, though, the value is supposed to be a Boolean telling whether the object was really deleted (i.e. if false, then another reference is still present.)