I followed the instructions in your link for the Mac. When I search for "Spotify" in the search field, three of the files it shows are related to ports (two files called PortIndex and one called Portfile). When I try to delete them it asks me for my password. Should I go ahead and delete them? Thanks.
I followed the instructions (correctly this time) and there was no change after re-installing, but I noticed something that I didn't before: the listed songs that can't be liked are local songs on my hard drive. I presume that's the reason the like feature is not available for them.