I do wish that businesses like Twitter would be more liberal (ahah, not like that) with their use of the banhammer for hate speech of all sorts, racism of all stripes, all behavior that is supposed to not be allowed, etc
That being said... unless the guy was actively engaging in prostitution or going after a minor, I'm not sure hitting on people violates the Twitter user agreements. Especially if done privately.
If he was engaging in illegal acts of any sort, yeah, ban him. If it's him hitting on people on his personal Twitter in private? Again, unless I missed that line, why would they ban him?