I had a similar issue, but with a ghost SMS. I tried all the tools and could not get rid of it. I ended up having to use
PPCPIM Backup or it may have been
Sprite Backup to backup all the PIM data cemail.vol and then restore it without restoring the text messages. You can do this but not restore the appointments, because I believe it overwrite the old data (or this can be selected). This may not be a viable option for you if you have a lot up upcoming appointments scheduled.