Need to have a different frame of time when your volunteers start collecting hours? Such as a school year. You can set new start dates. At this time it won't remove old hours, but in the volunteer list only the hours from the newly set start hours will show up.
1) Go to the Organizer Dashboard >> Settings >> More Settings
2) Select Other Configurations: Set your new Start Date (choose the month and date from the menu)
Set a custom milestone timeframe.
1) Organizer Dashboard >> Settings
2) Milestones >> Over what kind of time frame? select Custom.
3) Set your frequency and start date. This method will have a frame of time over which your volunteers will be required to submit their hours by.
If you need to have the hours from the previous year deleted, unfortunately there is not a way to efficiently do this yet. As the owner/admin you can delete the volunteers that you no longer need so their profiles and hours do not clutter up your site.