block registration page

Create a column like date/flag in database. For example, you have a field(IsRegistrationClosed) in Settings table. By default it'll be No, when you update that column with value Yes (manually/automatically) it'll prevent new registrations.

Check the value while creating new registrations. That's it. Do customize.

public bool IsRegistrationClosed()
{
     //Database related code
     //Code returns the Registration flag
}

public void btnRegistration_Click(Object sender, EventArgs e)
{
   if(IsRegistrationClosed())
   {
     lblStatus.Text = "Registration Closed";
   }
   {
      //Registration related code
   }
}