As you know using RNAi we are able to prevent gene expression. so why we do not use it to stop viral genes expression?
Delivering siRNA in vivo is a difficult prospect, but has been overcome in research environments and several commercial in vivo solutions are on the market see examples from Life Technologies here.
The bigger problems come from potential off-target effects. siRNA tend to be double stranded and both the 'guide' and 'passenger' strand can occasionally target multiple sequences that you did not intend. Additionally, siRNA tend to activate the body's immune system in ways that would inhibit therapy or cause excess inflammation and cell death. See the detailed review here.
In short, it is a good idea, and the current research in the field is working to overcome technical challenges, but we are not there yet.