Yes, every AI defense I see you get a few of those idiots, last time I played a castle defense there were even players from well known clans doing the same, racking up huge amounts of deaths with little kills, when some of the players on my team purposely blocked the routes they could jump from, they begun attacking our team mates.
Should be stricter rulings on Strategus, it shouldn't be for idiots just to sign up to battles just so they can jump off the wall, fail and die with little or no kills, rinse and repeat for an hour, they can go play Siege and do that.
Anyway, back on topic, the Roster should just have substitutes rather than accepting people after the battle begins and also it shouldn't remove anyone it has already accepted, if people don't show up then it can start accepting people from the substitutes. I know I'd be pissed if I got taken off the Roster after making the time to come on and play the battle.