Assalamu Alaikum
take them to a side, and say,
"i understand your intention and its noble may Allaah reward you for it, but it would be better to do it in a softer manner whilst paying clearer heed."
just an example, but say advise in your own words in a gentle manner. Its not too hard, its really easy to advise people if you have a smile on your face... for me when i smile at someone it seems as though they are completely relaxed. Then we can both freely speak, this allows for easy interraction/advice etc.
hope that makes sence, the sunnah of smiling is a great sunnah mashaAllaah
Assalamu Alaikum
