How big a deal is blocking copy-pasting on a website?

I'm working on a website on which the client wants to disable copy-paste.

It's not to do with security or passwords, but that the data on display is valuable if taken en masse (and almost no value if taken singly), though the user can only access the data one at a time. (There is no page a user can access with multiple items.)

I have advised that this is probably bad UX, and that it won't block screenscrapers or similar, but at the end of the day it's his decision.

How much of an impact does blocking this functionality have on a website? Is it likely to turn off users from engaging, or is it a non-entity and I am putting too much value in this?

If it's a big deal, do you have any recommendations on how to advise the client?

The client knows it won't stop even semi-skilled or determined individuals but wants to stop any casual attempts to retrieve data. I'm minded to just show him some of these answers in response.