After a bit of experimentation, I found a way to accomplish this. Though I doubt if it is the most elegant of solutions!
private void commentBlockToolStripMenuItem_Click(object sender, EventArgs e)
{
if (txtSQL.Selection.Text.Length > 0)
{
Range range = txtSQL.Selection.Range;
int f = range.StartingLine.Number;
int t = range.EndingLine.Number;
for (int i = f; i <= t; i++)
{
txtSQL.InsertText(txtSQL.Lines[i].StartPosition,"--");
}
txtSQL.Selection.Start = txtSQL.Lines[f].StartPosition;
txtSQL.Selection.End = txtSQL.Lines[t].EndPosition;
}
}