I'm with dynamox on this one, you will have to be a little more specific. There is no data allocation concept on Isilon, all capacity in the cluster is part of one filesystem (OneFS). You can limit the amount of capacity you show to clients via SmartQuotas if that's what you mean. SmartQuotas can easily be changed up or down, but there is no reclaim process involved.
You are probably taking about NFS datastores for VMs. Unlike block (SAN) datastore where some support of UNMAP is possible, on NFS datastores files can only be deleted/unlinked, or truncated in the best case. Any NAS system including Isilon makes that space available for reuse immediately (unless held back by snapshots). There can be, however, interesting interactions with the chosen provisioning scheme -- for vmware, check out the VAAI plugin for Isilon.