By default, any volunteer will be scheduled for any yearly or special service even if their preferences dictate that they can only be scheduled at their listed times and the service is not one of their listed times.
The setting that controls this behavior is located in the "Scheduler Settings" dialog, accessed by choosing "Scheduler Settings..." from the Schedule menu at the top of any open schedule window.
By default, the "Schedule any volunteer for any special service (regardless of his or her service preferences)" option is turned on. However, by clicking on the "Schedule only volunteers for a special service who have the service (or "[Any special service]") explicitly declared as one of their preferences" option, you can prevent any volunteers who do not have a special service listed as one of their preference from being scheduled for that special service.