Maybe disable softupdates, too?
Hmm...keep trying? I'm pretty sure I've deleted this file with a simple `rm .sujournal`, but I forget exactly how. My context is that in 9.0/9.1, FreeBSD reminds me politely that I can't dump snapshots while using soft updates. I *think* my solution was to disable softupdates journaling using tunefs, then disable softupdates, then mount the filesystem read-write, then delete the .sujournal file. For me, I could then dump a snapshot.
But it's been a while. My current solution is to not use snapshots, instead dumping filesystems that are mounted read-only, and otherwise let softupdates journaling do its thing.