First, I warn you that this approach of "poofing" variables might be a bad idea that could lead to difficult to debug issues further down the line.
Second, to do this efficiently you could use the structure output from whos() and ismember() to identify missing variables:
clear;
x = 1;
y = 7;
varsWanted = {'x','y','z'};
varsPresent = whos;
missing = varsWanted(~ismember(varsWanted,{varsPresent.name}));
