Someone with admin priv toggled that feature on for you. You can toggle it off.
Login in as admin
Launch admin console
Select the module EMAIL AND CONTACT from the drop down box at the top.
Select EMAIL SECURITY and scroll down to find clickable box for activating or deactivating " Add "Not a Member?" Links to Emails sent to Members via FreeToastHost."
REMINDER: There is super easy to access online documentation and info for all the FTH features available here:
support.toastmastersclubs.org/doc
And a link on the Members Only tab is featured on all FTH sites.