Question:

What is the process to disable the weekly auto-switch to the Lightning Experience?

Answer:

In Salesforce Classic:

1. Navigate to Setup.
2. Select Get Started under Lightning Experience Transition Assistant.
3. Select Go to Steps for option #3 Optimize Phase. 
4. Expand option two:  Motivate Users to Work in Lightning Experience.
5. For the Section labeled Switch users to Lightning Experience on a regular schedule, switch the On Switch to Off.

 In Lightning Experience, the steps would be mostly the same:

1. Navigate to Setup.
2. Select Lightning Experience Transition Assistant.
3. Select Go to Steps for option #3 Optimize Phase.
4. Expand option two:  Motivate Users to Work in Lightning Experience.
5. For the Section labeled Switch users to Lightning Experience on a regular schedule, switch the On Switch to Off.

Related Documentation:

Salesforce Help Documentation: Users automatically switched to Lightning after Summer '18