[LansdowneSt]

I'm not sure there is a toggle to turn it off except just eliminating the negotiation component of the draft altogether and having it so the picks sign automatically with the team that selects them. That's the "Enable Advanced Draftee Signing" toggle in League Settings, Rules, lower right. It defaults to negotiations.

The OOTP24 manual states:

Negotiation Results
The results of your negotiations can be positive, in which case the player joins your team and is placed in your designated for assignment area. Or they can be negative, in which case you will have to submit a new offer.

If you are unable to sign your draftees in the allotted time, they will sit out the year and will be eligible to draft again next season! If this happens, you will be awarded compensation draft picks in next season's draft for players drafted in the first three rounds. The compensation pick will be at the same draft number as the unsigned draftee plus one, so if a #12 pick in 2010 is not signed the compensation pick will be #13 in 2011. These compensation picks are not available for free agent compensation.