Minimum confidence level can be set at the command level, the profile level (via Profile Options) as well as globally by going into the Options > Recognition tab. These items can be found throughout the VoiceAttack documentation (search for, 'Confidence'). The VoiceAttack help file can be found in your installation folder and can also be accessed by pressing F1 (Help) when VoiceAttack is in focus.
Unfortunately, there is no way to exclude a certain sound from being picked up. You may want to try lowering your command weight (also on the Options > Recognition tab) if it's up too high (the higher the command weight, the more the speech engine will try to guess at what was said). I have a loud and clicky keyboard and I'm not getting any interference from it. Is it possible your mic volume is too high?
Edit - posted at the same time as Pfeil