Do wild ferns kill other flowers planted in a flower bed?

Over a long period of time they might smother other plants. Smother in the sense that they can grow fuller and taller depriving other plants of the light that they might need to survive.
Their extensive fibrous root systems could end up robbing neighboring plants of much needed moisture.

Keep in mind that this is typically a gradual thing occurring over time. It doesn't happen with a season or from one season to the next.

Nothing will happen overnight. If the bed starts reaching a point where the ferns are covering up the flowers, pull out some ferns. Easy as that. Probably when establishing a new plant, it would also be helpful to remove ferns from the area 10 inches or so around the new planting, just to decrease competition and give the transplant a head start.