Is there anyway to stop Windows 8 from restarting for an update?

This answer may be incomplete for current Windows builds

Windows has something called UpdateOrchestrator that runs a one-off task that will reboot as needed by servicing. Windows seems to repair this task when users try to nullify it, but borking permissions on the task's file seems to be a rational solution, and may now also be a requirement to prevent automatic restarts.

^{I had a machine spend a week analyzing a destroyed NAS RAID5 array across four 4 TB disks. It completed successfully and was ready to do recovery. I let the program sit there while I prepared to assemble the target resources. It automatically restarted while idling here.}

Original answer follows.

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Updates are always welcomed, and I like to think in software as complex as the likes of Windows NT indicate good health, or at least the desire for resilience. So I say assume good faith in claims of urgency. But if they're destroying your work, here's some Group Policy that considers your situation.

Tested solution in upper-segment Windows

Take a look at the Group Policy settings for Windows Update in gpedit.msc, in particular No auto-restart with logged on users for scheduled automatic updates installations:

They are in:

Local Computer Policy
|-Computer Configuration
  |-Administrative Templates
    |-Windows Components
      |-Windows Update

Change "No auto-restart with logged on users.." to enabled.

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Essentials segment Windows users

acatalept contributes the following solution for users of products offering the essential NT experience.

For non-Professional versions of Windows that don't have Group Policy Editor (gpedit.msc), you can accomplish the same thing as above in the Registry Editor. Navigate to this node (you may need to run it as Administrator):

HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\WindowsUpdate\AU

There, create a new DWORD (32-bit) value named NoAutoRebootWithLoggedOnUsers and set the value to 1. Then reboot for the change to take effect. See Pete's Stop Windows 8 Rebooting Due To Updates for steps.

Run services.msc, right-click the Windows Update service and stop it. This can prevent a scheduled restart even if you have been warned by Windows it will happen. Oddly enough, stopping the service with the NET STOP wuauserv command, does not seem to cancel restarting.
Of course, after a reboot, Windows can still restart. If you don't have access to gpedit.msc and thus can't apply Louis' more permanent solution, I suggest you go to Windows Update settings and set it to download automatically, but with manual install.