There’s at least one player who checks alliances. Meaning joins and leaves immediately. I’ve never seen him stay even if the alliance had an almost dying titan. Because of players like that some alliances changed from ‘open’ to ‘invitation required’ (I think it’s because of that…).
So my proposal is: when a new member joins, he can’t leave for one minute or two. I think it’s enough to reduce a lot the in-and-out visits.