Trump Does NOT Have the Sole Power to Forever Bar Ukraine From Joining NATO.

NATO Treaty

Article 10

The Parties may, by unanimous agreement, invite any other European State in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area to accede to this Treaty. Any State so invited may become a Party to the Treaty by depositing its instrument of accession with the Government of the United States of America. The Government of the United States of America will inform each of the Parties of the deposit of each such instrument of accession.

---

Article 12

After the Treaty has been in force for ten years, or at any time thereafter, the Parties shall, if any of them so requests, consult together for the purpose of reviewing the Treaty, having regard for the factors then affecting peace and security in the North Atlantic area, including the development of universal as well as regional arrangements under the Charter of the United Nations for the maintenance of international peace and security.

---

He could block submission of a treaty vote in the Senate to favor Ukraine join NATO while he is president.

He could ask NATO as a body to review the NATO Treaty for a possible amendment regarding Ukraine while he is president.

But that's all.