Hello euphie,
You can do this with template overrides.
You can insert a check (not just for rating but for other parts as well) to see if the user is not a guest.
Example code:
<?php $user = JFactory::getUser(); ?>
<?php if( $user->guest ) : ?>
do something (for example show text or image)
<?php else : ?>
here goes ratings code
<?php endif; ?>
HInt: make sure that $user is not defined elsewhere in the template.
Hint2: Use template overrides, read this
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates 
