It happens, but it's more complicated than just hitting "right" or "wrong" boss. And it's hard to organize a clan where CB damage ranges from 300K to 6M per key, without proper communication channels.

I try to advise my clan to hit the boss, from which they can get the best chest.

For example, you have a player who does damage to Normal CB from one key enough to get the top chest. But entire clan still can't kill the boss. This player will then go and spend the rest of his keys on Hard CB. Because there's no reason to waste keys on normal, as there's no chance to get a double chest.

At the same time, this player can send weaker team to Easy CB to do around 1.6M damage just to help the team. He shall not send the strong team as it will prevent 3-4 more players from getting chests.

Really need the function to block one or multiple clan bosses.  

Our clan has 3 clan bosses unlocked.  There was a time when we were easily killing CB 1and 2 and the rest damage was spent on CB 3. Everything was OK. 

Now the situation has changed:

1) Some of the players have grown stronger.  For example my damage was 3 kk per key and I had to do 2 attacks (=6kk) to get the top reward.  Now my damage is 4-4.5kk per key.  So I have no interest in doing more than 1 attack on CB 2. I'd rather hit CB 3 three times. 

2) Some players have left and others came in. 

So now we still kill CB 1. But CB 2 is often left with around 5kk HP. Would be great to be able to block CB 1 so that people only hit CB 2 and kill him for sure (and stronger players can still try out CB 3)

I agree, a blocking function would be very useful at clan bosses.