For quite a while now RIM has had a recent contacts option (Messages > Options > Messages Displays and Actions > Show Recent Contacts When Composing Email) available for email. When enabled, this option lets you choose recent contacts when sending emails, much like Gmail and other email clients. It's a pretty sweet feature if you don't like to have a crazy contact list but email certain people often. Sometimes it's a hassle if you have contacts pop up which you no longer want to see or just want to keep things a bit cleaner.
Fortunately there is a built-in option to do a bit of housekeeping. Simply head to your Messages app, then press the Menu key and choose Options > Recent Contact Management. From here you can add people to contacts, remove items from your recent contacts list or clear the list out entirely. If you use the recent contacts option, this can be a good way to further tweak things to your liking. *While it's been available since OS 6, this feature may not be available in all OS versions.*