There doesn't appear to be any way to remove a single client from a given line in the /etc/exports file. So unless there is some extra functionality built into API calls that manage NFS exports I don't see how WFA would be able to do something that isn't already possible via the command line. You should still get NetApp confirmation but this is my take...
I was pondering the possibilities myself... While there's no command that will allow you to explicitly pull out a single host from an export, you could use 'export -io' to re-write the export with the updated RW/R/Root hosts info. So, we do have the ability do pull this off assuming we could somehow capture the updated entries minus the host(s) entered.
Being new to WFA for the most part, I'm not quite sure of how this could be done. I explored the 'Update Export' command a bit to see if I could wrap my head around what's happening -
It appears that we are creating a new line for each security entry and appending the new host entry to that -
# get the rule for the provided export $securityRule = $export.SecurityRules
Looks like this one may have to be done via perl unless someone has any other way to spin this.
If I was going to do something outside of PowerShell, I'm thinking that I would opt for updating the /etc/netgroup and using netgroups for my exports instead. In my case, we would likely have to update several exports via WFA using the current command just to say add a new ESX host into the mix whereas a netgroup would make only one point of touch to update all exports associated with a particular VMware instance's datastores in a single flow.
I have been pretty busy getting things moving in the clustered ONTAP space and prepared for VMworld (Wait until you see the new stuff I am coming out with). I decided that since this seems to be a pretty hot request, that I would take a look at everything and figure out the easiest update. I have updated the Pirate Pack for Shared NAS for traditional Data ONTAP v2.0 to now allow for individual host management of NFSv3 Exports. I have also posted this command as a separate DAR - Command: Remove Host from NFS Export - 7DOT