block access to ec2 instance

Here are the steps to replace the key, Add or remove a public key on your instance.

You could also restrict SSH via the security group assigned to the instance.

You may want to use Sessions Manager to login to the instance which allows you to control access using IAM instead of SSH keys.

Check out this document - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/replacing-key-pair.html

Ensure SSH access over the public Internet isnt permitted. If it is, you will need to change the SSH key

Otherwise ensure said developer doesnt have any remote access to the environment.