Under Organization > Configure > Settings you can configure the security options, but I'm not sure if it's used for guest users too.
And, unfortunately, it is not possible to delete it automatically, you can only set the password expiration.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.