There is an option in the Scheduler Settings dialog that you can use to ensure that nobody is scheduled within a certain timeframe.
From the Schedule menu at the top of any open schedule window, choose "Scheduler Settings". Then turn on the "Always leave at least _ days between the services of each volunteer" and set the number of days. I recommend setting it to 8 days so that you prevent them from being schedule every week, but they can still serve every other.
Note that this setting applies to ALL volunteers. Turning it on may result in unfilled positions being left in your schedules if the scheduler is unable to find a way to fill the schedule while never scheduling anybody 8 days or less apart. In this case you can fill in the unfilled positions manually by double clicking on each unfilled position and selecting a suitable volunteer to fill that slot.