Might be difficult to return if you consciously reject faith in Christ.
Were those he was referring to actually crucifying Christ again?
No.
He is referring to those who witnessed that first hand but still rejected Him as yet future; as not the Christ the Law and the Prophets did say should come.
He is referring to their having rejected the Lord's continued miracles working in the Twelve, et al, as a further confirmation that He was the Christ.
It was over for them; there would be no repeat of His first coming; they would die yet in their sins still clinging to doctrines that had pointed to Him.
The writer of Hebrews is writing of those who tasted - saw all that, but rejected it, in contrast to the believing remnant of Israel he is writing to encourage to stand their ground in the midst of such a falling away of their nation.
Said believing remnant were in contrast to these...
Acts 7:51 Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers did, so do ye. 7:52 Which of the prophets have not your fathers persecuted? and they have slain them which shewed before of the coming of the Just One; of whom ye have been now the betrayers and murderers: 7:53 Who have received the law by the disposition of angels, and have not kept it. 7:54 When they heard these things, they were cut to the heart, and they gnashed on him with their teeth.