Someone gave me this advice in another forum, but unfortunately it didn't work for me as I only want to remove the offline areas from my device not delete the project off AGOS completely, but maybe this helps you?
If you just want to remove them from the device you should be able to go to your offline map in the FieldMaps app, click on the 3 dots and select 'Remove area'. If the offline area was configured in ArcGIS Online it needs to be deleted from there (arcgis.com/apps/fieldmaps/maps) to prevent it from appearing in the list of available areas. From FieldMaps in AGOL select the map you are working on. Go to 'Offline areas' -> 'Map areas' -> 'Manage areas'. Then click the checkbox next to the offline area and you should get an option popup to Delete it. This will remove the option to download that offline area for all devices. Hope that helps.