I don’t know what access is like for larger clans in terms of what ranks can access what items. You may try reporting them to Zendesk with a ticket, screenshots if you have them, and all information of when they joined the clan, when they left, and all the items they took. Player(s) might be temp-banned but I doubt you’ll get your resources back either way.
Definitely worth considering tightening permissions as best you can going forward.
This was a big problem in ESO for awhile; players join guild, wipe out the guild bank, and leave. Permission rules eventually got updated to try to curb this behavior.