Is there a simple way to temporarily grant network access to a single application? Is it possible to only allow one single application to access the network connection, excluding all other applications from generating network traffic. 

Streaming video or music locally over the router seems to be interrupted by i.e. Mail, Dropbox, or update services etc. -- Hence, I would like only applications such as iTunes or Spotify to be able to use the network connection exclusively when listening to music or watching a movie...
You could create a custom rule set to achieve this using Little Snitch:

https://www.obdev.at/products/littlesnitch/index.html