If you need to stop sales for a particular ticket type (i.e. Early Bird ticket) you can do so by editing the ticket type.
From your Event Home, go to the Details tab. Under the Sales drop down menu, select the Stopped option.
You will see in your Dashboard that Sales are Stopped on All Dates. If you'd like to turn your sales back on, simply change the sales status back to Active.
If your event has multiple dates and you need to stop ticket sales for all dates, click Advanced Event Settings under the event's Details tab and scroll down to Start/Stop Sales. Then click Stop Sales For All Dates. If you need to restart sales, click Start Sales For All Dates.