Log onto your Webmail account.
On your Dashboard, click onto the gear icon on the left-hand side to go to your Settings.
There will be a new icon on the left-hand side of the webpage, which will look like this:
Click on it to be sent to the contacts section on your account. On this page you should see a ticked-box beside 'Automatically add new contacts from e-mails I reply to'. Unticking this box will turn off new contacts being added from email replies.