If you imprison him, his words will still reach his followers. And if you send him into exile, you will only be sending him back to his followers: it will strengthen them.
Monday, 26 Safar, 13th year of the dawah / 1st year of the Hijrah · 1 BH
Details differ across the sources
The identification of the old man from Najd as Iblis rests on a weak narration.
The Quraysh held a secret meeting in the middle of the night at Dar al-Nadwa, their council house. Every clan sent a representative except the Banu Hashim, the clan of the Prophet ﷺ. Even Abu Lahab was not invited, because the plot was against his own nephew. They also left out Mut'im ibn Adi, who was protecting the Prophet ﷺ at that time.
According to a weak report, an old man knocked on the door during the meeting. He said he was a leader from Najd and offered to help them with his advice. Ibn Abbas said this old man was Iblis. The council feared that if the Prophet ﷺ joined the Muslims who had already emigrated, he would become a threat to Makkah. One suggestion was to lock him up in a house, and another was to send him into exile, but both ideas were rejected. Then Abu Jahal proposed that every clan send one man, so that they would all strike him together. His blood would then be on all their swords, and no single clan could be blamed. The Banu Hashim could not fight all the clans at once, so they would have to accept blood money.
The clans chose their men and sent them to the Prophet ﷺ's house. At the same time, Jibril AS came to the Prophet ﷺ and told him he must make the Hijrah, the emigration to Madinah, at once. That same evening the Prophet ﷺ spoke to Abu Bakr RA, and he left that same night.
Ibn Abbas reported that Qur'an 8:30 was revealed regarding this very plot.
Other events recorded in 1 BH. Their order within the year is not established; use the storyline above for the sequence.
If you imprison him, his words will still reach his followers. And if you send him into exile, you will only be sending him back to his followers: it will strengthen them.
You still haven't said the main point which everyone is thinking but no one has the guts to say: let us just kill him. We will do it in a way that nobody can get angry at any one tribe: let every single tribe send one representative, so that his blood is on everyone's swords and no one knows who killed him. Then the Banu Hashim cannot wage war against all of us, and will have to accept the blood money.
Spoken at the Quraysh's secret council at Dar al-Nadwa after their suggestions to imprison or exile the Prophet ﷺ had already been rejected.
This is the right decision!
وَإِذْ يَمْكُرُ بِكَ الَّذِينَ كَفَرُوا لِيُثْبِتُوكَ أَوْ يَقْتُلُوكَ أَوْ يُخْرِجُوكَ ۚ وَيَمْكُرُونَ وَيَمْكُرُ اللَّهُ ۖ وَاللَّهُ خَيْرُ الْمَاكِرِينَ
“And [remember, O Muhammad], when those who disbelieved plotted against you to restrain you or kill you or evict you [from Makkah]. But they plan, and Allah plans. And Allah is the best of planners”
Ibn Abbas reported this ayah was revealed regarding the Quraysh's plot to imprison, exile, or kill the Prophet ﷺ.
وَإِن كَادُوا لَيَسْتَفِزُّونَكَ مِنَ الْأَرْضِ لِيُخْرِجُوكَ مِنْهَا ۖ وَإِذًا لَّا يَلْبَثُونَ خِلَافَكَ إِلَّا قَلِيلًا
“They tried their best and they almost succeeded in getting rid of you from the land, but if they were to succeed, then they themselves would only last for a short while (before Allah destroys them).”
A Makki verse ("they tried to drive you out of the land") standardly tied to the Quraysh's plot to kill/expel the Prophet ﷺ at Dar al-Nadwa on the eve of the Hijrah -- not to Tabuk, a theory the DB's own battle-of-tabuk event explicitly rejects.