PostSharp builds IL at compile time and injects it. You cannot turn it off at run-time, and thus you cannot turn it off in PROD. However, you can very easily build an application setting that your PostSharp code reads and just return the default workflow when that flag is on.
At least this way you'd be able to stop any additional code from executing.
if (ConfigurationManager.AppSettings["YourSetting"] == "some_value")
{
args.FlowBehavior = FlowBehavior.Default;
return;
}