When one commits the sin of manifest heresy, the Church does not sever his membership, the one guilty of the sin of heresy severs himself from the Body of the Church by committing the sin.
That the one guilty of the sin of heresy severs himself is the exact point. He severs himself from membership in the Church by the very act of the public sin of heresy. It's called ipso facto.
“Public heretics (and a fortiori, apostates) are not members of the Church. They are not members because they separate themselves from the unity of Catholic faith and from the external profession of the faith. Obviously, therefore, they lack one of the three factors-baptism, profession of the same faith, union with the hierarchy-pointed out by Pius XII as requisite for membership in the Church (see above, p. 238). The same pontiff has explicitly pointed out that, unlike other sins, heresy, schism, and apostasy, automatically sever a man from the Church. ‘For not every sin, however grave and enormous it be, is such as to sever a man automatically from the Body of the Church, as does schism or heresy or apostasy‘ (MCC 30, italics ours).”
(Monsignor G. Van Noort, S.T.D., Dogmatic Theology, Volume II, Christ’s Church, 153)
“Certain sins – viz., apostasy, heresy and schism – of their nature cut off the guilty from the living Body of Christ…..It can hardly be denied that those who take up any of these positions – most evidently is this the case with the deliberate apostate – sever themselves by their own act from membership of the Church.”
(The Teaching of the Catholic Church, Volume II, Arranged and Edited by Canon George Smith, New York, 1961, Fourteenth Printing, p. 708)