Remove Salesforce Classic access for your organization

To remove their ability to switch to Salesforce Classic:

1. In Salesforce Lightning, go to Setup.
2. On the Quick Find box, enter lightning then click on Lightning Experience Transition Assistant.
3. On the Optimize Phase, click on Go to Steps link.
4. Click on Turn Off Salesforce Classic for Your Org to expand it.
5. Toggle the switch for Make Lightning Experience your org's only experience to On.

If you don't see the Lightning Experience Transition Assistant, the "Lightning only" permission has already been selected. 

Note: You cannot remove the Salesforce Classic access for specific Users if they have the "View Setup and Configuration" permission enabled for their custom profiles or through a permission set.

If your organization uses standard profiles, you can Create Permission Sets instead with a "Hide Option to Switch to Salesforce Classic" permission enabled.