Please note:

The league is currently set up to be a "legacy" league, in which teams are
allowed to carry-over some of players on their roster from the previous season
into the current season. Such leagues generally don't want to clear their rosters.

If you are a legacy league and you do want to clear your rosters, you can go
to the "Rollover contracts" page and enter "EXP" as the value of the new
contract for all old contracts. This will expire all existing contracts
and remove all players.

If your league isn't really a legacy league, or if you just want to start over
from scratch when entering your rosters, you can turn off the "legacy" status by
going to the "Line-ups and Rosters" set-up page and changing your answer to the
legacy league question (it's near the bottom of the page).