We also have a purge tool under C:\Adobe\LiveCycle8.2\LiveCycle_ES_SDK\misc\Foundation\ProcessPurgeTool that might be easier to use.
I know this tool and I tried it but it's a batch used to purge a lot of process intstances depending on the creation date and other stuff.
In the source of the batch i founded that it use the process Manager API to purge instances.
I need to purge a specific process instance and I only know the process instance ID of it.
The only method I know to purge a specific instance is this one (from the Process Manager API, the same used by the batch) :
ProcessManager pm = new ProcessManager(_factory);
But I don't know how to get the invocation ID of my process Instance.