On an up-to-date Debian testing system, lvremove fails sporadically when removing snapshots. The cause is not yet fully debugged, see Debian bug 549691. The symptom looks like this:
$ lvremove -f plana/snap_web Unable to deactivate open plana-domu--web-real (253:3) Failed to resume domu-web.
To work around this, use the following commands (works reliably, tested for a few days):
$ dmsetup remove /dev/mapper/plana-snap_web $ dmsetup remove /dev/mapper/plana-snap_web-cow $ lvremove -f plana/snap_web
Since they only remove the snapshot-mappings, this does not touch the data on the original LV at all.