I haven't had that message before. Send an e-mail to the admins, they can check it out for you.
Rosa

On the Gardening Forums it can mean that you've included the name of a vendor that they've banned from using the forums. There's an on-line nursery which operates out of Florida that is on the banned list - I think it was because they were marketing on the forum - and if you try to type their name into a posting it will bounce back as "illegal string". For example, you can't say "Wow, I just got a beautiful hibiscus at (name of store) this weekend!"