Go to your normal map's Alpha channel, and make it a medium to dark shade of grey.
Black works too but will cause your normal map to have practically no effect in game.
Basically, the darker your normal map's alpha channel, the less shiny your model will be. The brighter the alpha channel is, the shinier your model will be.
Another good way to control this more precisely, is to copy your Diffuse texture map and paste it into the Normal map (assuming they are the same resolution). The diffuse map that you paste into the normal map's alpha channel will turn greyscale. Lighter areas like skin, bolts, buttons etc will react to light more strongly, areas that have darker/more heavily shadowed alpha channel will react to light less and not be as shiny.
If your normal maps don't have an alpha channel, save them in DXT5 ARGB 8bpp, Interpolated Alpha, Generate Mipmaps - that should generate an alpha channel for you.
If you have any other questions feel free to PM me!