How can I simulate failure of an AWS Availability Zone?
This (slightly old in cloud years) question on Hacker News provides a few suggestions:


Remove the security groups from all instances in one zone, which should technically make them all appear 'down'
Adjust the VPC route tables to black hole the subnets in one availability zone
Disable an availability zone from the load balancer (docs)


You could also probably adjust the Network ACL attached to the relevant subnets to deny all traffic.

I'm not aware of an existing, available tool that does any of this automatically, though.