// get rid of any whitespace at the end
richTextBox1.Text = richTextBox1.Text.TrimEnd(' ', '\r', '\n', '\t');
// find length of remaining text
int length = richTextBox1.Text.Length;
if (length > 0)
{
    // get physical line number of last character
    int lastLine = richTextBox1.GetLineFromCharIndex(length - 1);
    // get character index of first character on that line
    int firstIndex = richTextBox1.GetFirstCharIndexFromLine(lastLine);
    // remove characters on last line
    richTextBox1.Text = richTextBox1.Text.Substring(0, firstIndex);
}