Forget the watermark in the corner, people will simply crop in a little bit and cut it out when possible. Put a watermark (subtle but present nonetheless) in an area where it could not be cut/cropped out. Also, it may be a good idea to have your clients get their names trademarked if they have not already done so. Instagram is quicker to respond to legitimate legal action of trademarks over copyright infringement, or at least that has been my experience thus far. Especially if you have the legal registered (or even applied) trademark proof that can be sent to instagram, they are more likely to respond to it and will shut down other people’s fake or copied accounts. One other approach to the whole situation which is a controversial topic is to inform the clients fan base that there are fake accounts out there, and request that if the fans see a fake account, that they will take the extra step of reporting the fake accounts. That way, more people are reporting the fake accounts and are in theory more likely to get shut down. Squeaky wheel gets the grease.