This is most likely included in the header of the theme you are using.
You can edit this by heading to Settings->Administrator
then clicking on the Header
file under the box titled Theme Editor
Once you have the header file open, look for and delete this line:
Code: Select all
<h4 class="login emphasis inline"><? PrintLogin(); ?></h4>
It will likely be found around line 22.
If this line of code does not exist in the header file, the login button may have been put in manually by someone. So if you can't find the PrintLogin() code, please post the url of the website you are working on here so I can take a look at the login button and gave you further advice!