This will change the password policy for all existing users:
net accounts /maxpwage:30
I only want to change the password policy for the one user. How do I do that?
Password expires only works with the policy
Password expires. So you need to set that too, which indeed is set for every user unless they have Password Never Expires set to true. But that only makes it so that they get to change the password after it expires, which is not what you want. They can also just edit their user and disable this if they want, given they're admin on that machine.
Instead, you can set an end date on the account itself and basically it can't be used after that date. This feature is unfortunately only part of a domain user, not a local user, but this really is the best way to do this.
An alternative is to create a script that disables the user, and setup a task using Task Scheduler to execute that script at a given time. This has its caveats too, as they can disable it if they find it.
If they access the machines through RDP remotely the easiest is to setup a 2nd port forwarding to the machines, probably through 1 pc with a different port, then you can just disable that port forwarder