Hey Urs,
You can change the text by modifying the language file.
COM_EASYSOCIAL_LOGIN_NO_ACCOUNT="Don't have an account yet?"
COM_EASYSOCIAL_LOGIN_REGISTER_NOW="Register now and start interacting with people from this community today!"
As for removing the registration button, are you sure you want to do that eh? How would the users register on the site? If you really need to do this, you could edit the file /components/com_easysocial/themes/wireframe/helpers/html/login.php and remove the codes below at line 54,
[gist]
<a class="btn btn-es-primary btn-lg t-lg-mt--md" href="<?php echo ESR::registration();?>">
<i class="fa fa-globe"></i> <?php echo JText::_('COM_EASYSOCIAL_LOGIN_REGISTER_NOW_BUTTON');?>
</a>
[/gist]