What is the problem exactly? A clanmember not being able to return the banner even if he/she wants to but can't do it because he/she equipped it and it's impossible to take it off? Or basically a clanmember not doing it because he/she didn't want to/wasn't told to do it before logging out? If the problem is the latter, all you have to do is tell that member this: "Hey, could you please take the banner off and put it in clan treasury? Thank you."
I'm sure that won't take them a minute to do it.
It's better to be silent and be thought a fool than to speak and remove all doubt. What's even better is to weaponize your words to destroy anyone who dares challenge you.