...I like what Audio stated because I was going to say something similar. I wasn't there so I don't know if you were right or not by kicking him off the set so to speak. In the case of Youth Explosions, it all depends on what the program is and who rehearsed with who and who is the house drummer and if the choirs that got invited brought in their own musicians etc. Youth Explosions are funny to me because it has a tendency on some occasions to turn into what band and choir is the best that night and yall know how drummers get.
I personally don't agree with that. Anyway, if you rehearse a song with the band and another drummer is sitting there wanting to play it, you have to speak up and let him or her know that you guys rehearsed for the song. If he doesn't get up, then there is something obviously wrong with him. You have to be wise in these situations or it could get ugly if that drummer isn't thinking right. Like I stated before, some of these cats have a chip on their shoulder. It sounds like you guys are dealing with some of those.
Sidenote:
...when we have Youth Explosions at my church, the Guests do not play unless we know them and their playing style. Another thing is that the equipment is expensive and my Pastor is strict with us on our equipment. Usually, the guests get a part and they sing a song that everyone knows.