You do not put any text in your
StringBuilder
object, so there is nothing to remove. And even if you did, the object is not used elsewhere so it serves no purpose. Your logic is the wrong way round it should be:
Dim sb As New System.Text.StringBuilder
For Each line In IO.File.ReadLines("C:\_Montix a.s. - cloud\iMontix\aaa.txt")
If (Not line.StartsWith(TextBox1.Text)) Then
sb.Append(line)
End If
Next