If you don't necessarily need to use the backspace key (simulate a key punch or how you want to call it) this will do the job of removing the last character in the textbox.
TextBox1.Text = Textbox1.Text.Remove(TextBox1.Text.Count - 1)
I even tested it for ya.
You might want to add an if statement in case there is no text in the textbox (it could cause an error).
If Textbox1.Text.Count > 0
TextBox1.Text = Textbox1.Text.Remove(TextBox1.Text.Count - 1)
End If
Hope this helped!