Unfortunately, there is no direct control mechanism to turn trading on or off for regular fantasy leagues. If you have a Best Ball league, there is an option in the General League Settings for disabling trades, but we don't have that available in standard leagues.
The best workaround for this is to set your Trade Deadline to Week 9. In addition, set the Trade Review time to the max of 3 days, and veto any/all trades that happen to come in.
In case you'd like to make this change after the 9th Week of the Regular Season, you can effectively use the Trade Deadline as a switch.
Hopefully, in the future, we can offer a specific option to disable trading for all leagues.