If the person is in earnst they will clear themselves. If the accused refuses, the request printed above. Excluding someone from the group is wrong. You must stand up for the coven member that is in your group even if you don't know them that well. They know the Laws of Karma and if they are lying they will be punished anyway.
People are innocent until proven guilty.
Personally, if I don't know someone well enough to tell if they are lying and if they couldn't meet with in a year I wouldn't include them in my coven. My solution would be very close to Lord Kale's. Only instead of calling the three of them into open coven, I would have them all called to council. They would all be advised before we would start that it is against the Laws to falsely accuse someone of something in council. That it would result in the accuser accepting the punishment.