Only a Guild Leader may ‘disband’ or delete an entire Guild at their discretion.
To disband a Guild, a Guild Leader would visit the 'Social' option in the main menu, and choose their Guild.
In the bottom right corner there is a Settings icon where the Guild Leader may change the MOTD, change if the Guild is 'Open' to auto-accept new members or 'By Request,' which requires approval, as well as disband the Guild. Disbanding a Guild will remove all players and Guild data from the leader boards.*
* Please note, once this is done by a Guild Leader, it cannot be undone. If the leader wishes to re-create a Guild, all players must rejoin.