If pulling the trigger means simply using his connection to get the subreddit taken down, banning him does not prevent him from doing that. He also made the comment outside of this subreddit so he did not violate any terms here.

If pulling the trigger involves intentionally baiting people to post term violating content to have them reported, those suspected posts should be removed with lower-than-usual bar for certainty given his previous statement.

If active enough moderation is not possible to remove such post, banning 4THOT would be punishing him for your/mod's failing in taking the responsibility of running a subreddit as has been agreed to do so when creating the subreddit.

If active enough moderation is not possible but you are simply trading keeping this subreddit to for unjustly censoring 4THOT, there shouldn't be any negative structural support against 4THOT by allowing criticism he can not counter if he chooses to do so.

Does this mean anyone can escape criticism by getting banned from the subreddit

More clarification on this: if a user gets banned for violating terms of this subreddit, they need not get protection from criticism. 4THOT was not banned for posting anything here that violates subreddit terms.