People don't underatand the basic networking of games unfortunately. If the difference is 100ms, and there is no way to track the cheating via conventional methods, your out of luck for banning people. There is no easy way to tell the difference unless you have all the data.
I am not sure what your proposed solution is, other than to ban people if they are too fast, even if they were actually that fast.
Even if you allowed everyone to cheat in the same way, people who live closest to the server would still win.
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