You can use the workprotect API's.. Refer this link
Well here you can use these API's in any of the hook methods. For instance on loading of the WebDynpro Component in the wdDoInit() method if you say something like
Then you cannot navigate away from this WebDynpro iView in the Portal until you say
But anyway the above link would help you acheive it. But ensure that in the Portal WorkProtect Mode setting you choose the kind of setting that you need.