[ovirt-users] How to clean up Storage Domain manually

We have a problem in our cluster where the storage domain was consumed almost totally so we proceed to do a release space by deleting vms and templates that are not longer needed. Despite of removal we got the following eerror message removing some assets "The Storage Domain may be manually cleaned-up from possible leftover". Also this problem is triggered when new vms are created. How can we perform a manual clean-up in the storage domain?