I'm sorry but the Bitnami Launchpad doesn't support reset the SSH key. That SSH key is generated when you first access the Launchpad and configured your account and all the deployments you launch use the same key.
What you can do is to generate a new SSH key in your computer and configure the bitnami user to use that new key and remove the default one. That way, the other user won't be able to access the instance anymore.
This section has information about how to configure the instance that way (you will need to follow the option B in the step 2).
Another option would be configure SSH to access the instance using the SSH key plus a password. Will that work for you?