I declined two requests back in the day. This was after a lengthy chat with CS who assured me that I would be no penalties. There were a series of check boxes and a place for notes but I can not remember if it was before or after the decline option and I'm not sure what's in place now.
Despite CS assurance my acceptance rate went down to 87% and I was informed I needed to "work" on it until it returned to the 88% level. Now I've read that metric doesn't count to your superhost status but still having to "work" on something that shouldn't be an issue is irksome.
If I feel I have to decline again I'm thinking about doing a live stream so if people want to check it out they can. We can have a discussion about it.
I actually used the adblock extension to hide the html classes that displayed the acceptance rate and my need to work on it. Little OCD perhaps but it worked.