@Evgeniya27
I don't know if that is true or not, but there is no need to decline an enquiry, so best just not to do it.
If you want to stop the guest from contacting you again, you can block them. You have to select the 'they are being offensive' option to do that. You can also report them. Or, you can just tell them no, and then ignore further messages.
With these types (and I have had a few enquiries like the one you describe), I just tell them no, and they usually go away. If they persist, I block them. I have never needed to decline an enquiry.