The default behavior of FSX is a binary one where flaps can only be fully extended or fully retracted. This means that when the spoilers are extended in flight and you press the [/] key to retract them, they will first fully extend and then retract. This is unavoidable FSX behavior that is very hard to avoid.
The best way to use the spoilers is to link them to an unused channel on your control unit (I used the little slider on the X52 Throttle) or to use lever in the VC. If you use the [/] key command you will have to live with this minor inconvenience.