You can remove the Sign Up button from an event, which allows you to have volunteers see the event but not sign up for it. This is useful if you need to post an event as information only.
To do this, simply edit the event and choose NO under "Allow Users to Self-Register." Also make sure that your Event Visibility setting is set at "Show this event when registration is disabled."
If you need to let volunteers to sign up for the event later on, go in and change the answer to YES.