First thing to do is collate all the complaints you have and check each name against the usergroups and write down which group they belong to, I suspect they are all in one group and a mod or plugin you have is coded wrong.
The fact that you have 200,000 members is great but you'll find that 90% of them have only posted maybe once and 90% of the remaining 10% have only posted a few times which is why it doesn't become an issue to many times
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