Why We Ignore the Commands of "Prophets"

It interests me that Agabus the Christian prophet never used his spiritual gift to tell anyone what to do. 

Agabus had the New testament gift of prophecy. Hey predicted a famine, which came true during Claudius' reign as emperor. But Agabus didn't tell the church how to prepare (see Acts 11:27-30). Agabus also predicted Paul's arrest, but he did not tell Paul what to do about it (Acts 21:7-14). This fact shows that Paul was not disobeying the Lord by continuing to Jerusalem.

This absence of commanding by a NT prophetically-gifted person is quite different from Old Testament prophets, who regularly told people what they should do. In fact, you disobeyed an OT prophet at great peril. This change, this absence of commanding by New Testament prophet, is consistent with the completion of Scriptural authority. Only the Scriptures, which were written and/or overseen by the OT prophets and the NT apostles, has the authority to command us. 

Christ gave the highest authority to bind and loose believers to the apostles, and to them only (Matthew 16:19). We still have the apostolic authority, not in the form of a man, but in the form of the Bible. 

This is why we should not fear or obey anyone who, claiming to act under the Holy Spirit's power, issues commands. A spiritually-gifted person can encourage us, comfort us, and build us up (1 Co. 14:3), but he has no authority to command us.