[SOLVED]New bie confused on changing or deleting plugin after composition.

Hello ,
I try to learn Qtractor and using in my projects enjoying and admiring.
In my last project i used some plugins and record sound thru my MIDI keyboard.
To do this i created a separate midi tracks and buses and used a synth (ACE fluintsynth) and a reverb (Calf reb) plugin in the busses.
In the synth plugin set sf2 soundfont (Yamaha c5..)
evrything was fine but afterwards i didnt like the piano sound and decided to refine it.
The problem is when i change the plugins even delete all the sound doesn't change .
I understood that midi tracks are just digital codes and consist some info about pitch, duration etc.
I expected that when i deleted the plugins it should be no sound. I searched the plugins elsewhere in case of i missed to delete but no way.
Im sure I miss something , would you please help me about it?
Thanks.
Edit : VMPK was set Sonivoweas synth and qtractor was connected VMPK , because i need to show the keyboard.
I set the VMPK 's driver and connection to Fluitsynth , im happy now.