The power to ex-communicate lies with the Guardian alone.

"...he feels that all National Spiritual Assemblies should bear in mind that this is the heaviest sanction we possess at present in the Faith, short of ex-communication, which lies within the powers of the Guardian alone, and is consequently a very weighty weapon to wield."

(From a letter written on behalf of Shoghi Effendi to the National Spiritual Assembly of Canada, March 3, 1955: Messages to Canada, p. 51)