It's been mentioned a number of times that YARG will never have a feature to delete songs from the song library; this is because unlike some other rhythm games, the song directories are entirely managed by the user, and we don't want YARG to ever be able to arbitrarily (and dangerously) wipe things on the user's disk.
#Tag songs for removal upon closing game
ember wind
There have been some talks about having something like an ignore list, but not something that will actually delete the song