Clash of Clans – Can Specific Players Be Attacked?

Yes, this "tiny castle" is in fact linked to their Clan Castle.

When in a clan, you can request troops from your clanmates. If you have troops in your clan castle, they will automatically defend your base if it is attacked and the attacking troops enter the clan castle's radius.

Alternatively, you can deploy these troops to bolster your own forces during a raid. They will show up in the troop selection bar with an icon denoting your clan's shield. Deploying them is an all-or-nothing affair.

Either of these two things will use up the troops in your clan castle.

The only explanation is that someone is inviting him. I would recommend demoting everyone but yourself down to member and letting it blow over. Then give back everyone's rank. This should discourage anyone from inviting him again.

It sounds extreme, but it is the only way next to disbanding your clan.

Also, There is no such thing as 'banning' someone from your clan. When you kick them out, they get a message when they try to join saying: You have been banned from this clan. However, they can just join the next day(24 hours). I would recommend changing your clan entry status to invite only too, that way you can reject his request if he was to make one.