Manually running the cleanup should remove it: UrBackup - Server administration manual
###11.3 Cleanup for servers with file backups with lots of files
UrBackup’s database is in a mode which enables high concurrency. Since the cleanup procedure can sometimes be bottlenecked by the database it may be advisable to switch the database into a mode which allows less concurrency but is fast for some operations for the cleanup procedure. This is not possible while UrBackup is running, so you should tweak the backup window such that you can be sure there are no backups running at some point. Then you can stop the server run the cleanup separately by calling
urbackupsrv cleanup --amount x
on GNU/Linux or on Windows:
cleanup.bat x
Where x is the percent of space to free on the backup storage or the number of Bytes/ Megabytes/ Gigabytes e.g. “20G” or “10%”. If it should only delete old backups use “0%”.