How should handle the client who asks you for unnecessary modifications?

Dig deep into his reason. Apply 5 why . Then ask him, how much is the current solution a barrier ?

Then finally, charge for modification.

That's way, you show him that he can leave with current feature. If he is not willing to, then he has to pay the price

Do it and charge for them. Period.

Charge for the modifications.
If they are truly bad, and you would feel bad doing that work, apply a multiplier to make it worth it for you.

Does he pay you? Consider making modifications if they align with your vision.
He doesn't pay you? Ignore, these feature requesters will ruin your runway.

You can try to explain why your design is the better option (in a constructive way). Why does it have the edge over their design? Will it convert more customers? What will your design achieve that their version doesn't?

If they still want it their way, document the conversation and do it their way (they are paying). If the client's design version doesn't work out, you can refer to your conversation and bill the hours it will take to do it still your way.