Quick and easy problem... I know how to set the timer and to set what ever you want to happen when the timer expires but I don't know how to stop the timer.
Setting the timer to 0 (e.g. SetTimer(1,0);) should end the countdown without running the IsTimerExpired condition. Alternatively, you could make the condition also dependant on a variable (e.g. if(IsTimerExpired(1) && MyInt == 0) {), and change it when you want to 'stop' the timer (i.e. if MyInt is any non-zero value, the condition won't run).
Cheers!