In table design, if you set the Data Type of the field to Number, Access
will reject any entry that is not a valid number.

If you want to block any characters that are not digits (including the
decimal point, negative sign, etc.) you could paste the function below into
a general module, and then set your text box's On KeyPress property to:
   [Event Procedure]
and add the code:
   Call DigitOnly(KeyAscii)

Sub DigitOnly(KeyAscii As Integer)
On Error GoTo Err_DigitOnly
   'Purpose:   Disallow any keystroke except 0 ~ 9, and backspace.
   'Usage:     In a text box's KeyPress event procedure:
   '               Call DigitOnly(KeyAscii)

   If KeyAscii < 48 Or KeyAscii > 57 Then
       If KeyAscii <> vbKeyBack Then
           KeyAscii = 0
       End If
   End If

Exit_DigitOnly:
   Exit Sub

Err_DigitOnly:
   MsgBox "Error " & Err.Number & ": " & Err.Description
   Resume Exit_DigitOnly
End Sub

Note that this code will have the side effect of disabling any non-digit
hotkeys while this text box has focus.