For a single select list box, set its value to Null.

I seem to remember that multi select list boxes are
troublesome.  Try setting it Row source to itself:
    Me.listbox.RowSource = Me.listbox.RowSource

Add this code to a standard module and call it from code in your form like:

  ClearListBox Me.lboDepts

Sub ClearListBox(pctlListBox As ListBox)
 '============================================================
 '     Purpose: clear all selection from a list box control
 '  Programmer: Duane Hookom
 ' Called From: Multiple
 '        Date: 2/21/2003
 '  Parameters: list box object
 '============================================================
   On Error GoTo ClearListBox_Err
   Dim strErrMsg As String 'For Error Handling

   Dim varItem As Variant
   For Each varItem In pctlListBox.ItemsSelected
       pctlListBox.Selected(varItem) = False
   Next

ClearListBox_Exit:
   On Error Resume Next
   Exit Sub

ClearListBox_Err:
   Select Case Err
       Case Else
           strErrMsg = strErrMsg & "Error #: " & Format$(Err.Number) &
vbCrLf
           strErrMsg = strErrMsg & "Error Description: " & Err.Description
           MsgBox strErrMsg, vbInformation, "ClearListBox"
           Resume ClearListBox_Exit
   End Select
End Sub