There are several sellers who are violating the TOS and I want to report them. In fact, I told the customer support about them. But they did nothing. Is there any way to flag them?
Only thing you can do is report them to Customer Support and wait for them to take action.
If you are messaging them, you can click on the Report button when you highlight their message, other than that you must contact Customer Support.
the report button only stopps spam. it does nothing else otherwise.