these are the snapshot files... you need to commit or power off that vm and take a clone.. of that vm then you can delete those files...
locate the vm which is associated with these files...
some times these files are still locked in the esxi host - refer the link http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=10051
and remove the lock for that file