How to delete arbitrary numbers of columns from a matrix in mathematica

I learned this method long time ago from Mr Wizard answer, it works well. But there are many other ways

a = RandomInteger[25, {9, 11}];
a // MatrixForm

ReplacePart[a, {{_, 4}, {_, 7}} :> Sequence[]];
MatrixForm[%]

The fasted way to do this is to use Part and construct the indices you need to take using Complement:

dropColumns[mat_?MatrixQ, columns : {__Integer}] := With[{
   columnsToTake = Complement[
     Range[Dimensions[mat][[2]]],
     columns
   ]
 },
    mat[[All, columnsToTake]]
];
a = RandomInteger[25, {9, 11}];
a // MatrixForm
dropColumns[a, {2, 5}] // MatrixForm

A = RandomInteger[25, {9, 11}]; 
A // MatrixForm
DeleteCol[Matrix_, indexlist_] := 
  Block[{internalMatrix = Matrix, i, k = 0, 
    internallist = SortBy[indexlist, Smaller]},
   For[i = 1, i <= Length[internallist], i++, 
    internalMatrix = 
     Delete[Transpose[internalMatrix], internallist[[i]] - k];
    internalMatrix = Transpose[internalMatrix];
    k++;
    ];
   internalMatrix
   ];
DeleteCol[A, {4, 7}];
% // MatrixForm
DeleteCol[A, {7, 4}];
% // MatrixForm