I do believe this was referring to a "flesh and blood man/men" of the first century. One that Paul couldn't have named openly (but spoke about with those of the Church at Thessalonica, when he was with them).
It makes sense to me that the "restrainer" was the High Priest Ananus ben Ananus. When the Jewish-Roman War broke out in 66 AD, Ananus led the peace movement against the zealots. Josephus said that Ananus “preferred peace above all things,” was “a shrewd man in speaking and persuading the people,” and “had already gotten the mastery of those who opposed his designs or were for the war” (
Wars 4.5.2). Ananus had restrained the zealots until 67 AD, when they appointed their own High Priest, Phannias (who was unqualified to be a High Priest) that the zealots used as their puppet (
Wars 4.3.6-8). The people of Jerusalem “could no longer bear the insolence of this procedure, but did altogether run zealously, in order to overthrow that tyranny…” (
Wars 4.3.9). In the speech of Ananus (Wars 4.3.10), he pledged to lead the people in an all-or-nothing attack against the Zealots, not sparing his own body. Ananus and his followers actually gained the upper hand against the Zealots, forcing them into the inner temple and gaining control of the rest of the city (
Wars 4.3.12), but their progress came undone because of the trickery of John Levi of Gischala (Wars 4.3.13-14).
Ananus and Jesus ben Gamaliel were both killed, along with other priests, during the
Zealot Temple Siege of February-March AD 68. I believe this was when the restrainer was taken “
out of the way.” Their deaths marked a significant turning point for Jerusalem, according to Josephus:
“I should not mistake if I said that the death of Ananus was the beginning of the destruction of the city, and that from this very day may be dated the overthrow of her wall, and the ruin of her affairs, whereon they saw their high priest, and the procurer of their preservation, slain in the midst of their city… to say all in a word, if Ananus had survived they had certainly compounded matters… And the Jews had then put abundance of delays in the way of the Romans, if they had had such a general as he was” (
Wars 4.5.2).
After their deaths, the Zealots were unrestrained. They “fell upon the people as upon a flock of profane animals, and cut their throats.” Others endured “terrible torments” before finally meeting their deaths. At least 12,000 died in that massacre (
Wars 4.5.3). Josephus described how the Zealots increased their wickedness because the peace-loving high priests were no longer there to hinder them:
“[T]he zealots grew more insolent, not as deserted by their confederates, but as freed from such men as might hinder their designs, and put some stop to their wickedness. Accordingly they made no longer any delay, nor took any deliberation in their enormous practices, but made use of the shortest methods for all their executions, and what they had once resolved upon, they put in practice sooner than anyone could imagine…” (
Wars 4.6.1).