First, click the Preferences button in the lower right corner of the SpiderOak application window, then select the 'Back Up' tab. If you don't want to back up files older than a certain date, check this box and enter a number of days. SpiderOak will not back up any files created before the specified number of days ago.
Please note that this preference only works for files that are not yet a part of your backup set. This preference will never purge files that you have already backed up, including historical versions.