Keep Users in Lightning Experience
Where: This feature applies to Lightning Experience in Professional, Enterprise, Performance, Unlimited, and Developer editions.
How: From Setup in Lightning Experience, enter Lightning in the Quick Find box, and then select Lightning Experience. In the Lightning Experience Migration Assistant, on the Set Up Users tab, turn on Keep Lightning Experience Users in Lightning Experience Only. For new orgs created after this feature is released, this setting is turned on by default, and the Switcher is removed. For existing orgs, this feature is turned off by default.
If some of your users still need access to both interfaces, use the Hide Option to Switch to Salesforce Classic permission instead. If your org doesn’t contain custom profiles, you can’t use this setting. Use the Hide Option to Switch to Salesforce Classic permission instead.