To prevent any further new registrations / prospective members for your club or fishery you need to disable online registration.

To do this go to the "Settings" tab on the top right of the screen then select "Club info" on the dropdown list. From here select the "Membership" tab and then tick the box (highlighted below) to stop any further Online registrations.

Anyone that has already registered interest as a prospective member will remain as such but this will prevent any new prospective members until unticked.