I have a 6+. I had the same issue. An album unchecked in my musiclibrary on itunes on my laptop kept syncing to my phone, even though in the sync on the summary tab 'sync only checked songs and videos' was selected and on the music tab 'sync entire library' was checked. Those settings should have resulted in only the checked songs in my music library being put on my phone. My solution was to do a sync with 'Sync Music' unchecked on the Music tab. This removed all the music from my phone... except when I checked my phone, the one album was still on it. On impulse, I right swiped the album, a delete icon appeared, and after selecting, the album was off my phone.
I checked 'Sync Music' on the music tab and synced again. This time the sync was correct, the album was not on my phone.
There is one caution: when I look at 'Music on my Device', some songs appear checked, with big circles to the left, but greyed out. I am not sure what that means. But the album no longer is on the phone - taking the music off my phone, manually deleting the album from my phone by swiping, and syncing the music back seems to have worked.
One strange thing was that while the issue was occurring, the album appeared under the sync 'Music' tab as 'manually added songs'. I don't know what they were doing there.