Can I delete a fill-up?

Yes. This can be done fairly easily:

-Tap the 'i' button at the bottom-right of your screen

-Select the vehicle from which you want to delete the fill-up

-Tap the "Edit" button at the top-right of the screen

-Press the delete "circle icon" on the left of the fill-up you wish to delete

-A delete button will appear which you can then tap to delete the fill-up