Yep.
Spooner also published manifestos (after the War) that the Constitution was only binding on those who actually signed it. All of whom were of course long-dead by then.
He had earlier supported guerilla warfare by southern slaves, and an attempt to forcibly rescue John Brown after his failed insurrection.
He was certainly not against violence, as such, just against violence for what he viewed as insufficiently pure motives.
"The Constitution of No Authority" is an interesting read. It's available via Gutenberg