How to deal with spam from contact form
Take a look at the subject line of the spam messages you are receiving, you will see that they all contain [spam]. This is added by Shopify as the messages pass through their system.
In your email program or even better within your web hosting (most hosts will do this for you). Add a mailbox rule to capture any emails which content [spam] in the subject line. Your rule can be set to delete, move or mark as spam.
This will keep the spam out of your inbox.