You cannot steal something given to you.

Also, the character ID number is the same. If you really are concerned about people taking things out of the FC chest, write down the ID numbers of these problematic people and check it every so often. Eventually if they change a name it can circulate around so people know who to run out for.

Or do not put extremely expensive items in a FC chest that is accessed by members you do not know very well.

Screen better, limit better. What would you propose SE do? They cannot have a "take items back from member" function, that would be silly.