Mark the dates you cannot play, and see the whole band's conflicts on one calendar.
Every member marks their own unavailable dates. The band's availability calendar shows everyone's at once, so the owner can see at a glance which dates are clear.
Unavailable dates belong to you, not to a band. If you are in three bands and you are away that weekend, you mark it once and all three see it — because your actual calendar does not change depending on which band is asking.
Marking a date does not decline anything by itself. It is information for whoever is booking, not an RSVP — those are answered per gig.