How to restrict a user from switching to classic from lightning and vice-versa in Salesforce?

Hi Kapil,

The permission set is always used to extend the features beyond to profile level in salesforce. If user has permission on profile so you can't remove the access for that using permission set.

Here if you want to prevent user from move back to lightning to classic then go to Lightning >Switch Users to Lightning Experience > there you can Control which users automatically move to the new interface and which remain in Salesforce Classic.

Second, If you want to restrict the behaviour for a user ,then disable at profile and do not assign any permission set that provides access to the lightning experience while assign permission set to users needing the functionality

Hope this helps you.