There's no re-lock alternative directly in the context menu for re-locking an unlocked BitLocker volume. So after a bit of searching I came up with a solution based upon two-three different posts.

  1. Unpack the content of this zip TODO which contains two files (One with the registry command to add the option to lock the BitLocker volume from the context menu and another that runs the BitLocker lock command manage-bde -lock -forcedismount <drive>).

  2. Copy the dismount-bitlocker.vbs to C:\windows\system32\ (If you have installed windows on a different drive you need to change the command.reg to mirror this).

  3. Run the command.reg file so that the new registry keys are installed.

  4. Right-click on a mounted BitLocker volume and ta-da!

Command.reg

Windows Registry Editor Version 5.00 
[HKEY_CLASSES_ROOT\Drive\shell\lock-dbe-rudi]
"AppliesTo"="(System.Volume.BitLockerProtection:=1 OR System.Volume.BitLockerProtection:=3 OR System.Volume.BitLockerProtection:=5) "
@="Lock BitLocker Volume"
"HasLUAShield"=""
"MultiSelectModel"="Single"
[HKEY_CLASSES_ROOT\Drive\shell\lock-dbe-rudi\command]
@="cmd /c \"c:\\windows\\system32\\dismount-bitlocker.vbs %1\""

dismount-bitlocker.vbs (see references for original source)

Args = ""
Last = Wscript.Arguments.Count - 1
For i = 0 To Last
 Args = Args & " " & Wscript.Arguments.Item(i)
Next
Args = Replace(Args,":\",":")
CreateObject("Shell.Application").ShellExecute "manage-bde.exe", "-lock -forcedismount " & Args, "", "runas", 0

Sources

Windows 8.1 re-lock Bitlocker unlocked drive

Download

Download code Bitlocker-content-cenu.zip