That's a good objection. I think, a major element is social control. If in a society it is deemed ok to yell and be smelly, people will yell and smell. But if you "make" this socially unacceptable, the result will be more hygiene and reservation. In the most cases more advertisement for soap should be enough.
The biggest problem for this kind of problem solving is when there are already pockets of parallel societies that you can't reach anymore, because they run a closed system. In this case, you need steamroller tactics like strict immigration or a mandatory military service where you teach the out-group how to wash and behave.
These out-groups are either completely harmless and just do their own thing (like the Japanese community in Frankfurt - you don't hear anything about them, but they are plenty). Or they are staying apart not by running an attractive parallel society, but by forcing its members to stay part of the group. In that case, you have all rights to go against the criminal behavior. As soon as the criminal and destructive parts are gone, you end up with A) a harmless out-group or B) the group assimilates on its own (like exile Persians usually do) or C) it's enough to use or enable social cohesion techniques.
Bottom line: If the people in charge are doing their job by going against destructive elements when they see them, I don't think there is a situation in which you need extensive force like strict immigration policies. The big problem is, neither the people in charge do their job (=politicians) nor the ones who should be observing them and tell the public about it (=journalists).
As I wrote in the answer to @yaanivapeji, you don't need to be a politician, but you need money. Then you can for example run a newspaper and tell everyone about negative developments.
Great points. I think that the reasons behind the bottom line problems you described are:
OK, this makes me feel depressed and helpless. :D