I agree with Kalle. Really the whole point of having a prefix is to use it with standby mode enabled. The solution is to use standby mode. That is not a quick fix, it is the correct solution.
If you only have certain commands that you think are subject to false positives then you could consider modifying the phrases for those particular commands, or increasing the minimum required confidence for those commands, but really if you intend to use an always on, open microphone then you need to use standby mode. If you want to switch to on mode for a few commands, you can set the idle timeout timer to automatically put you back into standby after a certain amount of time passes without any commands being recognized.
I would also like to point out to you that VoxCommando will only listen to one of your microphones. So if you have multiple microphones, it does not really matter, but you should make sure that the correct microphone is selected to be used. Normally it will be whichever microphone is set as the Windows default input device, but there is also an action in VC that you can use tot select which microphone to listen to.
http://voxcommando.com/mediawiki/index.php?title=VC.SetSpeechInput