Con artist class is basically made for newbies who still don't have access to the bank robbery requirement yet, so he steals bags from other bank robbers. There're ways tho that makes it impossible for anyone to steal your bag for a specific ammount of time which everyone who's a hardcore bank robber knows. Apparently you don't. However I'm voting -1 for this suggestion because the class if for newbies and shouldn't have any requirements of any kind. Next time beware of who you get close to.
If you are teaming with anyone just rob each other and con artist can't do a shit. If you are solo just use some techniques of escaping.
Whereas there is a suggestion made on restricting q abuse after depositing money bag. Hope it will be accepted. You can thereby rob him too if he doesn't have wallet.