Can someone tell me if it's possible to turn off our instance of Marketo temporarily?

You can just go to Admin > Salesforce > Disable Sync.

Then the two systems are disconnected.

when you turn it back on, the data will flow again, BUT Tasks created during the outage will NOT be sent over.

I would also do this after hours to avoid problems with lead flow.

Thanks but I'm not sure that will do what I want.  First off, we don't sync with a CRM, Second, we do  have a lof of assets in Marketo that are being used by our website .  Things like white papers, videos, and brochures.  Some of them gated by a registration form.  I'm looking for something that I can shut off that will demonstrate the effect on my web site when Marketo is disabled.  Is that possible?