Mass deletion of workflows

The easiest way would be to write a small little console app or even test project that references the K2 API (SourceCode.HostClientAPI, SourceCode.Workflow.Management). Here is an example that should get you most of the way there:

SourceCode.Workflow.Management.WorkflowManagementServer wms = new SourceCode.Workflow.Management.WorkflowManagementServer();

using (wms.CreateConnection())
{
wms.Connection.Open(K2ConnectionStringBuilder.ConnectionString);
foreach (SourceCode.Workflow.Management.ProcessInstance pi in wms.GetProcessInstances())
{
wms.DeleteProcessInstances(pi.ID, true);
}
}