It depends on what type of “button” it is. My guess is this is not a real button but a link being styled like a button. It will also depend on if it’s a button added with Visual Composer or one you added yourself by assigning the theme’s “btn” class. If you can link to the page where the button is I can look at the code to answer these questions, or if you know how to use your browser console you can check on this yourself.
Once you know the class of the button element you can add this to the Custom CSS field of your theme options:
.btn, a.btn { outline: none !important; }
button::-moz-focus-inner { border: 0; }
Whatever class you find on the button element, include it in the first statement above in addition to the classes I’ve already listed.