Unauthorized account changes

What should we do? Our church has 3 trustees, according to our by-laws their duties are “(1) hold in trust all properties of the church and all trust funds belonging to the church as authorized by church action,(2) be empowered by church authority to sign all deeds, mortgages, or other conveyances.(3)review, at least annually, the church insurance program, and act as agents of the church in the purchase of and handling of claims relating to buildings, vehicles, and activities.” 2 of the trustees went to our bank and changed the names on the signature card and who can cash in CDs and other changes. The church did not authorize these changes, so who is to be held accountable, the trustees or the bank employee? They and the assistant treasurer have also taken it upon themselves to "fire" our treasurer, the by-laws require a 30 day notice from either party. How do we rein these people in.