I need to remove fields from banned players. Is there a way to edit the .db file safely? Or someway to remove all fields owned by someone?

Yep its called sqlite manager.
then you can use SQL to delete them.

Or you can use the in game command /psdelete. Straight from the wiki:
http://wiki.sacredlabyrinth.net/index.php/PreciousStones#Commands

/ps delete [player/typeid] - Deletes all fields belonging to this player (use after banning) or that have the typeid.