Can't realize how to remove the booting image from RHEV ISO storage, can't find anything to perform such trivial operation.. what a shame.. In RHEV Manager I can only see a list of available images in "Store" tab.
The only current option is to access the ISO domain directly. You can go there using ssh, or
The ISOs are at /path/to/nfsmount/SDUUID/images/1111-1111-....111/
Where SDUUID is the UUID of the ISO storage domain (easy enough to find, but if in doubt, look at the API webpage)
Take care to make sure no VMs have the ISO you are removing attached. File deletion will fail if the VMs are up, but if they are not, and you remove the file, the VMs will fail to start up next time, so you'll have to go into VM settings and detach the missing ISO