If you nix something, you cancel or veto it. You might nix your little sister's plan to build a fire pit in the middle of the front yard.
In North America, to nix something is to forbid or put an end to it. Your aunt might nix your idea about a surprise party for your uncle, knowing he would hate it, and a babysitter might nix her charges' plan to stay up all night watching movies. Another, less formal meaning of nix is "nothing." This is also the oldest meaning of the word, from the German nichts, also "nothing," and pronounced in much the same way.