Can you bulk unapprove or archive emails?

Marketo supports Archive folders, which hide old emails and programs from search and selection to reduce accidental reuse, but archiving does not deactivate or unapprove them, so anything in use will still run unless you explicitly unapprove or deactivate it.
There is no native way to auto‑expire email approvals or unapprove assets via a workflow; this can only be done manually or programmatically via the Marketo REST API

Depends on how an email is sent and whether we’re talking about timed deactivation regardless of recent activity, or whether it’s okay to “keepalive” a campaign as long as it runs semi-regularly. If a trigger campaign doesn’t qualify anyone for 6mo it’ll be deactivated.

Beyond that built-in feature, as Pierre says, you’d need a separate process using the API. For example: add a program tag that signifies “expire campaigns/in this program 6 months after the last time the program was touched.” Use the API to get all the programs with that tag nightly and take call other endpoints to unapprove/deactivate.