以下代码是运用API实现的一种方法，还有一种方法是利用TextBox本身的KeyPress过程来实现的。

Option Explicit
DefInt A-Z 'DefInt (字母范围) 定义整型 -> 所有以A-Z字母开头的都是整型变量

Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hWnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Private Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" (ByVal hWnd As Long, ByVal nIndex As Long) As Long

Private Const GWL_STYLE = -16

Enum TxtBoxStyles
ES_UPPERCASE = &H8& '大写
ES_LOWERCASE = &H10& '小写
ES_NUMBER = &H2000& '数字
End Enum

Public Sub SetTxtBoxStyle(hWnd As Long, InputStyle As TxtBoxStyles)
Dim lngStyle As Long
lngStyle = GetWindowLong(hWnd, GWL_STYLE)
lngStyle = lngStyle Or InputStyle
SetWindowLong hWnd, GWL_STYLE, Style
End Sub

为了方便新手，我在这里将用KeyPress过程实现同样效果的方法简单说明一下。

Private Sub Text1_KeyPress(KeyAscii As Integer)
'显示出按下键的Ascii码
Debug.Print KeyAscii
'不给输入 "a"、"s"、"d" 这三个字母
If KeyAscii = 97 And KeyAscii = 115 And KeyAscii = 100 Then
'当按下asd这三个按键时屏蔽
KeyAscii = 0
End If
End Sub