No, let the leaves die back on their own, not necessary to cut them down. If your unnamed Agapanthus is evergreen, certainly don't cut them down - if it is actually Agapanthus africanus, will need to be brought under shelter for winter.

Applying washed seaweed or finely chopped banana peels can bring out bloom in Agapanthus, too. Also helps with cold hardiness.

will banana peels attract slugs?
they LOVE grapefruit skin

Not in my experience, Bram, but we don't have as many slugs here as in the UK! :)