Sounds like you want to remove y-values greater than a certain value.
One way:
idx = find(y>1500);
% now remove the y values and the corresponding x-values
y(idx) = [];
x(idx) = [];
Note that if your x-values were originally evenly-spaced, the above will alter that spacing. Not sure if that is important. There are other things you can do that will preserve the equal spacing. For example, you can set y-values greater than 1500 to NaN (not a number)
y(y>1500) = NaN;
