Simply turning off Track Changes leaves the changes in the document file,
even though they may be hidden by setting the view to Final. Many people
were unhappily surprised when they sent such a file to other users, who were
easily able to see the markup. For this reason, Microsoft made it the
default that markup is shown when saving or opening a document.

You can turn off this setting in Tools > Options > Security, but for your
own safety I recommend that you leave it turned on.

The way to prevent changes from appearing is to ACCEPT or REJECT all of them
before saving the document. On the Reviewing toolbar, the Accept Change and
Reject Change buttons have down arrows that open dropdown menus, where
you'll find Accept All and Reject All.

If you want to keep a record of the changes but send out documents that
don't let others see the markup, make a copy of the original. Accept or
reject all changes in the copy and send only the copy, keeping the original
to yourself.