Very weird, and I agree, they're probably getting your best offers but they would only get those if they watch or view your items, so I don't know why they're getting so upset.
I'd say just block them and don't worry about whether they still get anything, as you'll never hear from them again (blocked buyers can't message you, though they won't know that they're blocked unless they try to buy something).
An FTC complaint could only really go against eBay, you're not managing the list of who receives messages, so you're not liable.