synagogue - when is shushing in a shul permited, when forbiden and when obligated

In my shul there is someone who shushes people that talk. He believes that he is obligated to do this. I understand that there is a prohibition against embarrassing people publicly and that sometimes you are obligated to protest publicly on a violation.

My question is (please give sources so that he will respect the answer more): In this shul people usually talk (there is no minhag not to talk) and the people that are talking are not davining. When can one attempt to quieten the talkers and when is it forbidden?

1. There is no minyan (<10) and the chazan started to davin (befor yishtabach)



2. There is a minyan but the minyan is holding before baruch sheamar


3. The minyan is holding before shma


4. The minyan is holding in shma


5. The minyan is holding in shmona esrai


6. The minyan is holding in chazoras hashatz


7. The minyan is holding before krias hatorah


8. The minyan is holding during krias hatorah


9. Between aliyos


10. After krias hatorah

thank you very much