Certainly a user can copy just the contents of a cell without copying other parts of it too (borders, fonts etc). But how to get them to do that—or how to prevent them from doing anything else—I don't know. You can "protect" a worksheet, and you can even protect certain cells or ranges on a worksheet. And that protection can be granular; you can allow users to sort rows but not add new rows, for instance. There seems to be the ability to allow a user to change the format without changing the contents. But I don't see an option under the Protection feature that would allow the reverse, to change the contents but not the formatting. I'm going to guess you can't get there from here.

I suppose there's a way to do it in VBA—there almost always is. But it would probably involve changing the formatting back whenever someone changed it, which might be overly complex, and more work than it's worth.