unable delete snapshots created by NDMP backup

We have some issue with the NDMP backup on FluidFS. It looked like the problem was some snapshots created by the backup jobs and didn't get deleted after the jobs were completed. I tried deleting them manually from the Storage Manager and it complained they were locked by the NDMP backup. There is not backup job running and I also rebooted the backup server. Can one advice how I can remove those snapshots?

Thanks