I don't quite understand what has / has not worked, but:
1) If you go to Notes > Tags, open the tag in question, and long tap (press and hold) on an item, the display will change to have checkboxes on the left side, and you can check the box next to each that you want to delete and then tap the trash can icon in the icon bar. NOTE: I had to do this twice before the annotation was deleted - and it was entirely deleted (not just the tag, but the highlight too). The first time, it had no apparent effect (it asked for confirmation, but the annotation was still in the list), the second time, the annotation disappeared from the list and when I went back to the verse in question, the highlight was gone.
**IMPORTANT: DO NOT force a sync (Settings > Advanced Options > Resync All Annotations). Doing this pulls everything from the church's servers and disregards un-synchronized changes on the Android (it assumes there's a problem on the Android and you need to pull annotations fresh from the servers, so it overwrites the Android). Let the sync happen naturally.
2) If after that it still doesn't work, I would try going onto the Notes feature of LDS.org and deleting the highlights from there. (It looks slower there as you have to select one, delete it, repeat. But this may be needed if they keep coming back.)
3) If that doesn't work, the only other thing I can think of is to hack the Gospel Library database, which doesn't work as well as you might like. Then you'd have to sync (to convince the church servers it's gone). (Or you can try different sequences / combinations of power-cycling, clearing the cache, and deleting the app data - which would require re-download of content and annotations.)
FWIW,
Liz