Unfortunately, there is no direct control mechanism to turn trading on or off.
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.