Open Ports on Google Cloud Load Balancer
You can't add deny rules to GC firewall. The default policy is Deny. You can only add allow rules - allow everything you need and let everything else get rejected. 

Since the ports you need to block are allowed by default, you simply need to remove them. Check the name of the default rule:

gcloud compute firewall-rules list [NAME …] [--regexp=REGEXP, -r REGEXP] [--filter=EXPRESSION] [--limit=LIMIT] [--page-size=PAGE_SIZE] [--sort-by=[FIELD,…]] [--uri] [GLOBAL-FLAG …]


and delete it with:

gcloud compute firewall-rules delete NAME [NAME …] [GLOBAL-FLAG …]


You may check here for more detailed explanation on how to handle Google Cloud firewall.