Sadly, there is no command in-game to delete 1v1 PVP Arenas. Although, there is still an option which is the following:
- Repair Server -> 1v1 PVP
This is the only solution to delete 1v1 arenas. If the arenas you want to keep, outnumbers the arenas, you want to delete then we suggest you keeping them. That is, if want to get through all the trouble re-creating the arenas.