It's tough, since most of the characters are essential, and this guy would straight up kill them. Technically, the Thieves Guild will probably die off on it's own just because your character doesn't join them. Beyond that, tackling any criminal activity around Riften to keep them contained is about the most you can do, unless you're on PC, in which case there's a quest mod for imprisoning/destroying them.
It's tough, since most of the characters are essential, and this guy would straight up kill them. Technically, the Thieves Guild will probably die off on it's own just because your character doesn't join them. Beyond that, tackling any criminal activity around Riften to keep them contained is about the most you can do, unless you're on PC, in which case there's a quest mod for imprisoning/destroying them.