Disable "automatic maintenance" in Windows 8

You can do it by task scheduler. For this open the task scheduler by typing task on the Modern UI under the Apps and you will find the Task Scheduler at the left side click it and open. Now go to following path

Task Scheduler > Microsoft > Windows > Task Scheduler

Here you have to right click on the Idle Maintenance & Regular Maintenance and select the option disable, that's it!

If you want to enable it again then just do the reverse.

Automatic maintenance does too many functions, like the updating of Windows software etc. so that turning it totally off is not advisable (or possible).

To complete the answer by nhinkle, one could at least disable it from waking your PC.

This is done in Action Center / Maintenance section / Change maintenance settings, uncheck "Allow scheduled maintenance to wake up my computer".

Together with setting the scheduled time to late at night or a time that it won't be a bother (3 AM is the default), is about the maximum possible solution.