When you send marketing emails (non-operational), unsubscribe text and links are appended to the bottom. You can change the defaults. Here's how.
Edit the Unsubscribe Message
Under Admin, click Email.
Edit the Unsubscribe HTML and Unsubscribe Text versions to your liking and click Save Changes.
There you have it. Make sure to test! You don't want your marketing emails to have broken unsubscribe links.
You can customize the position of the unsubscribe HTML in your email by using tokens.
Default Unsubscribe Text
If you ever need to revert to default system unsubscribe, copy/paste the following: