A = [
1 1
2 2
6 3
12 2
4 5
9 6
14 1
5 1
6 9
8 10
11 11
12 3
9 13
4 6
10 15];
% group size
n = 5;
% index of max value, within each group of n (5)
[~,idx] = max(reshape(A(:,1),n,[]),[],1)
% convert to index in A
rows_to_delete = (0:numel(idx)-1)*n+idx
% delete the rows of A
A(rows_to_delete,:) = []
