At what point do you feel it is justified to delete a forum? and have you ever had to?
I used to be big, BIG into anime. So I started a forum in late 2013. It was called Anime West. It had 80 members that joined from some forum promotion websites and some gaming forums. It was all good for awhile.
Common problems were None of my staff members were helping in the way that I wanted them too. Talking to them and having meetings in the staff threads didn't seem to make a lick of difference. Another issue as with all boards was advertisement and arguing with people why YOUR board is special and better than the other guy's board. Another issue was money for advertisements and finding new ways to promote fresh activity on the board.
Eventually my worldly obligations caught up to me and I just didn't have the time for any of it. So I deleted the forum. Everyone was disappointed but they moved on rather quickly.