Yeesh, I didn't realize Smart was so thin skinned (I probably should have given his history). Those comments aren't even that bad. Sure, it's degrading to tell another man to get on his knees, but it isn't that big of a deal, that's just childish playground level smack talk. That is exactly the type of thing a player should be able to ignore, and not being able to ignore it is a sign of weakness, which the opposition is looking to exploit.
Personally, I think the best reaction to something like this is to just shut them up with your play. The more you engage hecklers the more they feel validated. Smart just let the fans of every other team know this stuff gets under his skin, and he can probably expect a lot more of it going forward. He better get a thicker skin real fast.
But that aside, the league is within their rights to control fan behavior. So if the league wants to prohibit this type of thing that is up to them. I might not agree that this needs to be prohibited but I wouldn't be bothered if they do.