assuming that A is your matrix with A(:,1) the column of your datenums, then simply do
% Determine which rows to delete
% (note that the "hours" of datenum are behind the decimal point)
% hence we do mod 1 to just get that number
rows_to_delete = mod(A(:,1),1)<11/24 | mod(A(:,1),1)>14/24;
% Delete them
A(rows_to_delete, :) = [];
Or if you are using a newer version of matlab you could get to some extra comfort and use datetime
% Convert to datetime
new_time = datetime(A(:,1), 'ConvertFrom', 'datenum');
% Determine which rows to delete
rows_to_delete = hour(new_time)<11 | hour(new_time)>14;
% Delete them
A(rows_to_delete, :) = [];
