When guests log in to their Spotify account over the wifi, the bar gets thrown out. I would like to have in place a setup where I could still use Spotify, but have it blocked for our guests. We use a standard wireless router from our telephone provider (Telefonica), the brand is "Comtrend" (Unfortunately I'm pretty blank when it comes to access hardware and such). Suggestions?
How much control do you have over your WiFi / LAN settings? Is it a combined WiFi router, or is it just an access point?
Some WiFi routers have built-in firewalling. If you can block the great unwashed from accessing 18.104.22.168/22 then you'll block off Spotify's IP range. You can exempt yourself from the block by giving your computers static IPs that aren't blocked by this rule.
The cheapo comtrends that telefonica most probably supplies to its customers cannot do what you want.
If you are serious then pop down to your local tech store or browse amazon and get a new device. Most of the ones you have to buy have what you need which is basically filter the incoming and outgoing traffic.
I'm not too familiar with spotify but I'd venture to guess that it uses standard web-browsing ports to tunnel traffic. Unless you want to invest in a next-generation firewall that is application aware like a Palo Alto Networks or even the latest Sonicwall, there is usually a way around.
You could try other methods, blocking by DNS entry...
What kind of firewall is in place today?