Here an example on how to disable  or set the calendar previous dates to not selectable.

ASPX:

<asp:Calendar ID="Calendar1" TodayDayStyle-Font-Underline="false" 
              runat="server" 
              SelectionMode="None" 
              OnDayRender="Calendar1_DayRender" >
              <DayStyle Font-Underline="False" Wrap="False" />
</asp:Calendar>

RELEVANT CODE:

  protected void Calendar1_DayRender(object sender, DayRenderEventArgs e){
        if (e.Day.Date < DateTime.Now.Date){
            e.Day.IsSelectable = false;
            e.Cell.ForeColor = System.Drawing.Color.Gray;
        }
  }

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