Can't see how implementing such a feature would require "lot of work". Telling people to stop changing the music is not really possible when hosting a house party, people won't obey it anyway. Setting a screensaver password would restrict me to use the computer for other tasks in the meantime.
A common problem/annoyance for me (and surely many others) is when people you don't want changes the song or rearranges the play queue. This happens all the time at house parties and similiar situations, everyone wants "their" song to be played, and "guarding" the computer isn't very easy at such occasions. For me, as a host, I would therefore love the ability to use Spotify in some kind of "Lock mode" – you choose a playlist or construct a play queue, and then while playing said music you lock Spotify down. This function would mean that the chosen music is played in the background, but in order to skip to next/previous track, change song or rearrange the play queue the users password would be required. Pausing the music and changing volume would still be possible in this mode. If we had a Lock mode in Spotify, playing music at parties would be a lot more fluid, uninterrupted and pleasant experience!