This question has already been discussed in the Hirhurim blog, in which R. Student argues that no matter how we understand contemporary semicha today, a rabbi's semicha can be revoked by the mentor who gave him semicha. As an example of this happening, R. Student cites R. Rakeffet's biography Bernard Revel, which describes how R. Revel [first president of Yeshiva College] revoked a particular student's semicha:
When a Yeshiva graduate refused Revel’s request to leave a position which had both mixed pews and a mixed choir, his ordination was revoked. Revel wrote to a graduate on September 19, 1933: “It grieves me to inform you that since you refuse to leave Temple…where the sacred laws of traditional Judaism are violated, I urgently request that you return the conditional document of ordination that you received from the Yeshiva. The basic purpose of the Yeshiva is to guard the sanctity of Jewish Law in this land. If you will not return the document of ordination, I will be obligated to publish newspaper announcements declaring the nullification of your ordination.” The rabbi did not heed Rabbi Revel’s request, and the Yeshiva publicly announced the cancellation of his ordination and proclaimed that “one can no longer rely on his answers to inquiries of Jewish Law.”
Furthermore, there are various institutions which have begun providing a disclaimer that they can revoke the semichot they give. Yeshivah Pirchei Shoshanim declares as much on their site:
It is at our discretion to block access and terminate either the course or revoke one’s Smicha of any participant if they do not adhere entirely to all precepts of the TORAH and the teaching’s of its Sages while maintaining the high level of personal and moral integrity that is demanded of one who is either learning for Semicha or has achieved that designation.
Similarly, after an Israeli rabbi admitted to stealing Torah scrolls, the Israeli Chief Rabbinate revoked his semicha, creating a new precedent:
Rabbinate representatives said that this is a precedent, and from now on this will be the policy towards any Rav who bears an official document and is convicted of offenses.
Regarding rabbis convicted in the past, the possibility is examined
whether a certificate from the Chief Rabbinate will be denied them
Hagr"y Metzger [Ashkenazi Chief Rabbi]: "The council wanted to send a
message that a rabbi who transgresses does not only transgress a
halachic, moral or value prohibition - but this may jeopardize his
continued tenure and status as a rabbi in Israel."
In addition, a certain Rav accused of falsifying addresses and license
to hold Chuppahs, was summoned to a hearing and it will be decided if
he will be denied a certificate too.