You would use a regular expression for this. I'm far from an expert with regular expressions so I usually give them a miss but this seemed like a rather easy one so I figured I'd give it a go. This worked for me in my one simple test:
vb.net Code:
  1. RichTextBox1.Text = Regex.Replace(RichTextBox1.Text,
  2.                                   "\d-\d-\d",
  3.                                   String.Empty)
The "\d" in the pattern stands for any numeric digit.