How to prevent user to enter occurrence characters in TextBox using VB.NET

Public Function checkCharacters(ByVal str As String) As String
        Dim temp As String = ""
        Dim f As String = ""
        Dim s As String = ""
        For i = 0 To str.Length - 1
            For j = i + 1 To str.Length - 2
                If str(i) = str(j) Then
                    temp += str(i).ToString()
                    If temp.Length >= 4 Then
                        Return temp.ToString()
                    End If
                Else
                    temp += ""
                End If
            Next
        Next
        Return ""
    End Function

    Private Sub btnCheck_Click(sender As Object, e As EventArgs) Handles btnCheck.Click
        Dim str As String = txtStringtoCheck.Text
        Dim sd As New StringsDelimiter
        Dim result As String = checkCharacters(str)
        Try
            If result.Length >= 4 Then
                MsgBox("Please entered repeat characters " & result.ToString())
            End If
        Catch ex As Exception
            MsgBox(ex.Message())
        End Try
    End Sub