Hi @Mary53,
I can see how this is annoying and I am not sure why you would receive them. As you can see by the image below, I put in dates where you are parially booked and it looks like it will allow me to send a message but when I choose send, it says I cannot as the dates are not available. You can even see that it warns me that "Those dates are not available".
If you do get these, the best thing you can do is reply that it is not available and then archive the message.

David
Superhost Ambassador ~ Host Club Community Leader ~ Experienced Co-Host