I guess it depends on what exactly you mean by "on demand".
If you mean that when a certain pattern of traffic flow is detected you want to stop processing the iRule, you can use the event disable command (
Click here) or the return command (
Click here) to manage this on a per-connection basis.
If you mean administratively removing the iRule, the GUI is probably the easiest way, unless you want to develop a custom iControl application that would be able to toggle the use of iRules on a particular virtual server.
Denny