What you see there are probably your "Most Recently Used keywords", which are stored in the configuration file THUMBS10.INI.
You can either just ignore them, or use a different method to enter keywords (e.g. press Ctrl-K / Strg-K ).
If you really, really want to get rid of them, maybe you can use a utility that edits ini-files from the command line to remove the [MruKeywords] section from thumbs10.ini. I have never used it, but it looks like INIFILE (
link) can do this for you.
So, what you would do is create a batchfile that first runs INIFILE to clear the keywords and then it launches ThumbsPlus (or, you could create 2 batchfiles -- one for each of your databases!). Somewhat like this (you'll have to change the paths and filenames for your setup) :
@Echo off
INIFILE thumbs10.ini [MruKeywords] /remove
"C:\Program Files\Thumbs10\Bin\Thumbs10.exe" "D:\Data\FirstDatabase.tpdb8"
Finally, you would make a shortcut (icon) on your desktop for each batch file, and from then on you can launch ThumbsPlus with the desired database with a single double-click.