Dear dum,
No, a saint cannot be de-cannonized. Saints from the early days of the Church were accepted as saints by popular acclaim as in the case of the apostles and early martyrs.
However, saints can be taken off the liturgical calendar and a number have been. There are various reasons for this. Often as the result of research, the historicity of a particular saint is found to be weak. This would only be the case for saints from antiquity. Later saints have gone through a canonization process. If a saint has gone through the involved process of canonization, the matter of historicity isn’t likely to be a problem.
Fr. Vincent Serpa, O.P.
