I suggest that when someone leaves a clan, his/her clan castle is emptied. I imagine it would be a lot of programming to have those troops returned to whomever originally donated them, so maybe just have the troops deleted instead.
This will hopefully reduce the number clan-hoppers that just show up for donations and encourage players to stick around in a clan for a while.
I get that some might just attack before leaving a clan but I'd let supercell figure out how to make this work:)

