There’s no standard mass request cancellation feature in WorkflowGen, and it may be a bit risky to provide such a function.
However, there is a way to do this by creating a script that does an HTTP post of the following URL to cancel each of the 200+ requests if the user have the permissions to cancel it (like using a WorkflowGen administrator account for the HTTP request connection):
http://mywebserver/wfgen/show.aspx?QUERY=PROCESS_INSTANCE_CANCEL&ID_PROCESS_INST=99999&ACTION=ABORT&NOTIFY_PARTICIPANT=N