REMOVE 'SCREEN LOCKED BY' IN ARD

• Make sure Apple Remote Desktop is not running, and navigate to the folder containing the application.
• Right-click on the application and choose Show Package Contents.
• In the window that opens, navigate to Contents/Resources/English.lproj.
• Find the file LockScreenTask.strings and create a backup copy in a safe place.
• Open the original LockScreenTask.strings in TextEdit.

You should see the following:
    /* Localized versions of task strings */

    taskString = "Lock Screen";
    taskDescription = "Lock Screen";
    longTaskDescription = "Lock screens on target computers";
    actionString = "Lock Screen";
    lockedByString = "\r\rScreen locked by %@";
    lockedByStringNoText = "Screen locked by %@";
    defaultPictureName = "Lock Screen Picture";
    // Task progress
    kLockScreenRequestingOne = "Locking screen";
    kLockScreenRequestingMany = "Locking screens";

Change the line
    lockedByStringNoText = "Screen locked by %@";
to
    lockedByStringNoText = "";

Now "Screen locked by" will not appear when you lock the screen of a client computer if you don't type a message, but it will still appear if you do.

If that's not sufficient, replace the line
    lockedByString = "rrScreen locked by %@"
with
    lockedByString = "rr";

Now the "Screen locked by..." message won't appear even if you send them a message.