It has been discussed before, but it’s a good one, so thanks for raising it again.
itsi mentioned this is planned in an old thread.
“Our intention is to make a calendar feature as you have suggested where you can block out dates and last-minute requests.” -itsi
Others have also discussed the general topic of host calendars and related features here and here.
I would love to see at least a basic version of this feature soon. Besides making things more convenient for everyone, I think it could quickly improve reply/acceptance rates, make “part-time” hosting more viable (therefore more hosts), and generally help people waste less time on the app writing and reading requests that aren’t going to work out.