Same opinion. You have right to say: "User ABC is doing this, overloads our servers, so I don't want him to mine here anymore." Being fair enought sending him e-mail "We're switching you off". But not paying out what he mined before you banned him, is scam. If you had some kind of rules saying "No pool hopping or be banned" somewhere on your website, then fine. But not this way.
There are easy ways to make pool hopping useless.