I wouldn't advise you to turn this off and it is currently not possible to disable this from the composer but I guess you could hide the button by editing the file /components/com_easyblog/themes/wireframe/composer/toolbar/default.php

[gist]
<div class="btn-group" data-toolbar-view data-type="cover">
<?php echo $this->output('site/composer/toolbar/cover'); ?>
</div>
[/gist]

And change it to,

[gist]
<div class="btn-group t-hidden" data-toolbar-view data-type="cover">
<?php echo $this->output('site/composer/toolbar/cover'); ?>
</div>
[/gist]