In 2 Thessalonians 3:15, how should a believer who disobeys Paul's instructions be admonished?

Answer: As a brother

Paul says to note such a person and not associate with him, so he may be ashamed, yet not regard him as an enemy but admonish him as a brother.

“Take note of anyone who does not obey the instructions we have given in this letter. Do not associate with him, so that he may be ashamed. Yet do not regard him as an enemy, but admonish him as a brother.”— 2 Thessalonians 3:14-15 (BSB)