Remove all Roles from a user MVC 5

User manager has a method Task<IList<string>> GetRolesAsync(TKey userId) which

Returns the roles for the user

And also Task<IdentityResult> RemoveFromRolesAsync(TKey userId, params string[] roles) that

Remove user from multiple roles

so combine the two to achieve what you want

var roles = await UserManager.GetRolesAsync(userid);
await UserManager.RemoveFromRolesAsync(userid, roles.ToArray());