Using Bitlocker, how do I lock a drive after I have unlocked it without having to restart?

Whenever I boot up the computer my drive that is encrypted with BitLocker (win7) is locked. When I access the files on it I need to enter my pword. When I need to close the drive however, it stays unlocked and I can't seem to find the feature to lock it back up (if there is one). The only way I know how is to reboot the machine again. Any help would be great! Thanks.