Maybe this helps:
Dim I As Integer
For I = 0 To CheckedListBox1.Items.Count - 1
If CheckedListBox1.Items(I).IndexOf("Whatever You Are Looking For") <> -1 Then
CheckedListBox1.SetItemChecked(I, True)
End If
Next
Or use the ChecktIndices propery like this(I know it's C# but it's the same):
foreach (var item in checkedListBox1.CheckedIndices)
{
checkedListBox1.SetItemChecked((int)item, false);
}