Thanks Lizzie for info on how I can get to the support line of Airbnb.
I ended up--I think--figuring out a way to solve the problem without contacting the support line. Here is what worked for me, arguably only one time, let's see if it will work the second time:
1. You want to decline, and want to choose the reason 'unavailable, block these dates on my calendar', but the decline button for that won't work.
2. Choose another reason, such as 'i don't feel comfortable with this guest'. You'll see that the decline button becomes clickable. Don't click on it, and instead go back to the 'unavailable' option.
3. This time, the decline button becomes clickable. Click on it. And done.
Best,
Tewodaj