The manual doesn't appear to mention anything about removing these entries, and pressing delete(which works in some dropdowns) doesn't do anything.
It's a workaround, but if it bothers you enough, you can go to "%localappdata%\VoiceAttack.com\", in the "0.0.0.1" folder under the "VoiceAttack.exe_Url_" folder, you'll find "user.config".
Make a copy of this file as a backup before you modify it, and close VoiceAttack.
Open the file in notepad(or any text editor that doesn't mess with the markup), and find the entry for "TargetMRU";
It'll look something like this:
<setting name="TargetMRU" serializeAs="String">
<value><?xml version="1.0" encoding="utf-16"?>
<ArrayOfString xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<string>wwwwwwwwwwwww</string>
</ArrayOfString></value>
</setting>
If you have no other entries in there you want to keep, you can just remove the entire section, otherwise only remove the "<string>wwwwwwwwwwwww</string>" line.
Save the file and start VoiceAttack, if you don't get an error message about user.config, everything else should remain the same, but the "wwwwwwwwwwwww" entry will be gone.
Otherwise, overwrite user.config with the backup copy you saved and try again(note: if you have issues, don't post the whole file here, as your registration data is included near the top).