How to stop programmatly the threads when a Form is running [on hold]

I need to know how to stop programmatly the threads launced from a Parallel class while the Form is running.The Form is updated by the function Form.ActiveForm.update() but the problem is to give the control to the Form from an user's input.Hence during the functioning the FORM don't respond.How can be it accomplisced? The problem is simple: give the possibility at the user to interact with the FORM when the code is running.The Task Manager has this possibility.Actually for me is sufficient push a button and to send thr notification at the programm in order to stop the running.

thanks in advanced.