You cannot stop it. It is doing what it is meant to do. Just sow more dill seed.

FataMorgana

Dill, like the vast majority of plants, responds to the length of the day (night) to trigger the reproductive cycle. This is called photoperiodism. Dill is considered a very sensitive, long-day plant. Once the hours of darkness are reduced in the early summer, you can't stop the dill from flowering.