I am afraid it's currently not possible to "Delete all Spam" like you would do with Comments. That's a good suggestion though, and I can see how the situation can be frustrating for you right now.
I have passed on your feedback to our developers.
In the meantime, I am afraid the only quick solution would be to delete all Spams directly from your database. You can find them all in the posts table, like so:
SELECT * FROM
post_type= 'feedback' AND
Please note that your posts table might have a different name, based on the database prefix you chose when setting up your site.