Saying no to neighbors

I own a vacation rental property. Neighbors relatives wanted to rent last summer. I charged half price because the week was open anyway, and I could not go. I worried that they thought this was too much because they delayed paying, and then paid a little less than I asked for. This year they asked again. Again, the week was not filled. I offered a couple of days that I could to go for free. I could not make an excuse because they know the house is empty. The truth is I do not want to do this. I have no security if anything breaks, and I really like to keep my tenant relationships business and get my full price. I did this out of fear of causing hard feelings if I charged too much or said no.
I am also worried that my family will be mad if they find out because they don,t even use it for free.

Also, I do not like the condition in which they kept the house. How can I say no now, after setting this precedent already?